Organic pollutant adsorption on clay minerals

Organic pollutant adsorption on clay minerals

7

Jean-Franc¸ois Lambert Laboratoire de R
eactivit
e de Surface, Sorbonne Universit
e- CNRS (UMR 7197), Paris, France

7.1

Pollutants? Definitions and scope of the chapter

In a chapter on pollutants adsorption on clay minerals, it seems a logical first step to define what is meant by ‘pollutant.’ A more precisely defined term is ‘xenobiotic’: this word refers to any anthropogenic substance that is not normally present in a given ecosystema (or not in significant quantities) and is introduced into it as a consequence of human activities. ‘Pollutant’ is a more emotionally charged name, as is indeed the whole issue. It is impossible to separate the scientific data on environmental pollution from economic, political, and sociocultural aspects, as shown by two examples among many (see Scheme 7.1): – –

Daminozide is a growth regulator sprayed on apples. It is also used to improve their visual appearance. In 1989, legitimate concerns about its carcinogenic effects were hyped by the media, resulting in a panic known as the ‘Alar scare.’ At the time of writing this chapter, a protracted political and legal battle has concerned the banning of glyphosate, an organophosphorous derivative of glycine bearing a phosphonate group (brand name Roundup), as a pesticide in Europe.

Organic pollutants may be more or less susceptible to biodegradation, that is, to the transformation by living organisms into harmless compounds, and the most resistant ones are called persistent or recalcitrant pollutants. Partial transformation of pollutants may give rise to long-lasting ‘bound residues’ (Gevao et al., 2000). The most harmful effects of pollutants arise when they are ingested by humans, and by plants and animals in the food chain. Therefore, the noxiousness of a particular molecule will depend on its bioavailability, that is, the fraction that is available for incorporation into living matter. Bioavailability depends on the distribution between the various compartments of the ecosystem. Fig. 7.1 is a highly schematised view of compartments in soils. Estimating this parameter correctly is crucial for establishing a rational basis for environmental pollution norms. It is sometimes argued that existing norms based on overall pollutant contents in the soil are unnecessarily stringent because a large fraction of the pollutant stock is so strongly adsorbed as to be unavailable. However, detailed a

Originally, ‘xenobiotic’ referred to substances foreign to an individual organism, but the generalisation to ecosystems is now generally accepted.

Developments in Clay Science, Vol. 9. https://doi.org/10.1016/B978-0-08-102432-4.00007-X Copyright © 2018 Elsevier Ltd. All rights reserved.

196

Surface and Interface Chemistry of Clay Minerals

Scheme 7.1 Two xenobiotic molecules, diaminozide (Alar) and glyphosate (Roundup), that have given rise to much controversy.

Fig. 7.1 Exchange phenomena between the different compartments of soil.

research indicates that such claims are oversimplified because the bioavailability is highly dependent on the nature of the adsorbents and their interaction with the pollutant molecules (Boyd et al., 2011b). Clay minerals are one component of the mineral phase, probably the most expansive one from the point of view of surface area. The exchanges of xenobiotic molecules between them and the aqueous phase (groundwater) are adsorption–desorption phenomena. Thus, both the quality of drinking water and the effective toxicity of pollutants depend on their adsorption on clay minerals, interacting with the other exchange processes in the biotope. Therefore, it is important to study the adsorption of pollutants on clay minerals for at least two reasons: (i) to understand their mobility and bioavailability in soils and other real-life systems and (ii) to control them through the use of clay minerals as adsorbents, in the frame of water purification, as amendments applied in the field or as controlled release formulations (Yusoff et al., 2016) used to ‘smooth out’ the concentration profiles of pesticides as a function of time, avoiding a sudden concentration peak followed by washing out.

Clay minerals are already used in water purification cartridges or as lining materials in landfills (Koutsopoulou et al., 2010), to avoid leakage of dangerous substances. In both cases, their use has emerged empirically, and the retention of organic pollutants is only one of the desirable properties of clay minerals: another one is the reduced hydraulic conductivity they afford. The fact that these technologies are already developed constitutes a strong incentive for the fundamental understanding of clay mineral– organics interactions. Other domains of clay science may overlap with the adsorption

Organic pollutant adsorption on clay minerals

197

of organic pollutants, such as inorganic–organic nanocomposites. For example, nanocomposites composed of clay minerals and epoxy resin are certainly relevant for the study of bisphenol A adsorption because those resins are formed by polymerisation of the bisphenol monomer, sometimes directly in the clay mineral interlayer space (Lan et al., 1996). This chapter is centred on adsorption into natural or ion-exchanged clay minerals, with only limited coverage of organoclays (clay minerals exchanged with surfactant molecules, making their interlayer space hydrophobic). Of course, research on adsorbing materials is not limited to naturally occurring materials, and more or less complicated formulations have been tested for pollutants fixation, in which the clay mineral is combined with one or several other components. This chapter does not attempt to cover this field, nor the adsorption of organic molecules by soils of which clay minerals are only one component; the task would certainly need more than one book chapter. A recent review of pollutants adsorption by organoclays has been published by Park et al. (2011). A more general review of pesticides in soils is published by Arias-Est
evez et al. (2008). The reference Handbook of Clay Science contains two chapters closely related to the topic of the present work (Nir et al., 2013; Yuan et al., 2013). There are also articles dealing with specific classes of compounds, for example, antibiotics in soils (Thiele-Bruhn, 2003). As regards the type of pollutants, this chapter is limited to simple organic molecules; some natural toxins that may be regarded as pollutants are complex proteins. Proteins adsorption on clay minerals is covered in Chapter 8 of this book. Heavy metals, including radionuclides, are also important nonorganic pollutants, and a special chapter is dedicated to them (see Chapter 5 of this book). The role of clay minerals with respect to organic molecules is not limited to their immobilisation, reversible or not. They can and do catalyse different transformations through heterogeneous catalysis. These may be beneficial, as when adsorbed pollutants are mineralised to CO2 in photocatalytic processes. But sometimes, reactions initiated on clay minerals cause transformations to even more dangerous compounds, such as dioxins. Conversely, adsorption may stabilise or protect pollutants against transformations that would occur if they were dissolved in the aqueous phase. Again, this may be undesirable, or desirable, in the case of controlled release formulations in which the pesticide should not be inactivated before being made bioavailable. The question is briefly covered in Section 7.5, but it has clear overtones in Chapter 9 of this book on Clay Mineral Catalysts. Throughout the text, many ‘semitrivial’ names (see Section 7.2.2. for a definition) or trade names are used for polluting substances. The reader is invited to refer to Appendix, which contains a lexicon of pollutants whose adsorption on clay minerals has been studied (not exhaustive).

7.2

Classification of pollutants

The number of compounds released into the environment by humans is enormous; for example, it is often claimed that more than 10,000 different substances are used as dyes alone. There are several ways to classify pollutants.

198

Surface and Interface Chemistry of Clay Minerals

7.2.1 Classification by origin Fuels, detergents, industrial surfactants, dyes, pharmaceuticals (pharmaceutically active compounds), including veterinary pharmaceuticals, antibiotics (which are not only pharmaceuticals but are included in animal food), personal care products (PCP), and explosives are all released as waste, either at industrial sites, or for some of them, after use by the consumers, in a more diffuse manner (wastewaters). Even micropollutants, released in small quantities, may cause significant health problems, either because of locally high concentrations, or because of their high potency. Thus, endocrine-disrupting compounds have been the object of considerable concern because they can interfere with hormonal pathways, even at a very low concentration. They include contraceptives, but also substances manufactured for quite different purposes, such as bisphenol A, used in epoxy resins. Such substances whose noxiousness has only been recently recognised are sometimes called emerging pollutants (e.g. perfluorinated compounds). The seriousness of the problem is likely to increase as effluents from wastewater are more and more often recycled, due to the shortage of drinking water in many locations. In contrast, pesticides are spread mostly voluntarily in the field. They include herbicides, insecticides, fungicides, bactericides, nematicides and general disinfectants. Some substances are also applied to crops to regulate plant growth and development. In addition, some molecules may be considered as xenobiotic because they represent a disruption from the normal state of the ecosystem, yet have a natural origin, such as wildfires, volcanoes or volatile organic compounds produced by vegetation. Some industrial organic molecules may not be very harmful, but their transformation can result in extremely toxic byproducts, for example, when pyrolysis of chlorinated molecules forms polychlorinated dibenzodioxins and dibenzofurans.

7.2.2 Classification by chemical nature A cursory look at the structures represented in Appendix will convince the reader that extremely diverse molecules are included within the group of organic pollutants, and have been studied in the context of clay mineral adsorption. Due to their structural complexity, most pesticides and pharmaceuticals are not designated by their official IUPAC name. Most often, a ‘semitrivial’ denomination is used to identify the active chemical, such as glyphosate (reported in Scheme 7.1) for N-(phosphonomethyl)glycine. The term ‘semitrivial’ means that the name ‘glyphosate,’ while being simpler than the official, IUPAC-sanctioned nomenclature, is still chemically motivated, as it alludes to two functional groups of the molecule, the glycine moiety at one end, and the phosphate group at the other end. Thus, ‘glyphosate’ is the name under which the substance is found in most databases, but for the general public, it is better known by the trade name Roundup (actually Roundup is a mixture that contains surfactants, in addition to glyphosate as an active ingredient). Often the denomination is an abbreviation of the IUPAC nomenclature name, for example, DDT for dichlorodiphenyltrichloroethane (this compound may also be called clofenotane) or DCMU for (3-(3,4-dichlorophenyl)-1,1-dimethyl urea)—its

Organic pollutant adsorption on clay minerals

199

tradename is Diuron. There is no one-to-one correspondence; the same substance can have many different trade names, and a trade name may not correspond to a single chemical, but to a commercial formulation of several active species, for example, the herbicide Resolva is actually a combination of diquat (its IUPAC name is 6,7-dihydrodipyrido[1,2a:20 ,10 -c]pyrazinediium dibromide!) and glyphosate. Often, several pesticides or drugs are modifications of the same basic structure. This is quite logical because it is more likely that a new, active compound will be discovered by modification of an existing one than by random, de novo synthesis. Therefore, they may be loosely grouped in classes or families based on the recognition of common structural features. This classification is quite arbitrary, depending on what features are considered as relevant. For example, the basic units of phenylureas and acetanilides have a lot in common. A given molecule may well belong to two classes. Picloram, belonging to the picolinic acid class, is also an organochlorine—a rather broad class potentially including all molecules with at least one CdCl bond. Metsulfuron-methyl is both a sulfonylurea and a triazine. Broader distinctions are made according to basic chemical properties, the most general one being between ionic and nonionic molecules (Gevao et al., 2000); the abbreviation NOC is often used for nonionic organic compounds. Within ionic compounds, one further distinguishes between cationic, acidic, and basic ones, which may seem puzzling. In fact, ‘cationic’ designates molecules that have a permanent positive charge, independent of the protonation state, such as diquat and paraquat (see Scheme 7.2), or the dyes methylene blue (MB), acriflavine, crystal violet, and malachite green. Therefore, they are supplied as salts containing compensating ions, often chlorides. On the other hand, many molecules have moieties that take part in acido-basic equilibria, such as carboxylic acids (–COOH) or amines (NR3). Their speciation will be pH dependent. In the first case, if the pH is inferior to the pKa of the carboxylic acid/ carboxylate couple (2–4 most often), the carboxylic acid form will predominate, and the molecule will be neutral, and if the pH is superior to the pKa, the deprotonated form will predominate, and the molecule will be anionic. In the same way, when the pH is inferior to the pKa of the ammonium/amine couple, the molecule will be protonated, and thus cationic. Of course, many molecules have more than one acido-basic moiety, and their pH-dependent speciation will span more than two different charge states. For example, the antibiotic ciprofloxacin has two pKa, and its speciation is reminiscent of that of amino acids: the predominant form is a cation at pH  6, a zwitterion (bearing

Scheme 7.2 Paraquat, a molecule with pH-independent cationic charge, here as the chloride salt; atrazine, a basic molecule, cationic at low pH; mecoprop, an acidic molecule, anionic at high pH.

200

Surface and Interface Chemistry of Clay Minerals

both one positive and one negative charge) between pH 6 and 9, and an anion at pH  9 (Wang et al., 2011). Tetracyclines have four pKa, so that as the pH increases, the net charge of the molecule goes from +1 to 3 (Figueroa et al., 2004). Molecules of all these classes (cationic, acidic, basic) are fairly soluble in water (e.g. glyphosate, 12 g L1 at 25°C). Other pollutants, such as the PAH (polyaromatic hydrocarbons) or DDT, do not have either a permanent charge or acido-basic groups. They may still differ in hydrophilicity, and therefore in water solubility, due to the presence of polar groups, for example, parathion, with NO2, PdOd and P]S groups, is about 100 times more soluble than pyrene (a PAH). Attempts have been made to correlate the adsorption of pesticides on clay minerals with their hydrophilicity (Polati et al., 2006), but using a single parameter has only little predictive value. Many functional groups present in pollutants have good ligand properties, allowing them to form coordinative bonds with metal cations (not only transition metal ions, but also alkaline and alkaline earth ions). This is particularly true for amine and carboxylate groups, but also, to a lesser degree, for carbonyl, ether, sulphonate, and even chlorine groups. If several such groups are present in close vicinity and correctly oriented, the molecule may act as a polydentate ligand, stabilising the metal–ligand complex by a chelate effect. Ligands are, by definition, Lewis bases. Lewis bases are not necessarily strong Br€onsted bases, that is, they do not have to be able to undergo complete proton transfer from a Br€ onsted acid. However, hydrogen bonding may be viewed as partial proton transfer, and therefore those groups that can act as ligands are also susceptible to act as H-bond receptors. Conversely, Br€ onsted acidic groups can act as H-bond donors, and so can XdH groups that have no measurable Br€ onsted acidity. One last type of functional group often present in organic pollutants is aromatic rings. They are a defining feature of PAH, and are often present in dyes too, whose spectroscopic properties are then the consequence of highly conjugated π electron systems. At first sight, they would seem to be typical hydrophobic groups, unable to establish anything but van der Waals interactions, and thus susceptible to segregation out of water-rich regions (see Section 7.3). But in fact, the electrons in aromatic orbitals can give rise to a specific coordination to metal cations, where the ‘naked’ cation is nested above the aromatic ring to maximise orbital overlap (Dougherty, 1996). In such a complex, all carbons participate in the bonding to the same extent, so that the ‘hapticity’, that is, the number of C atoms bound to the cation, is equal to the number of atoms in the ring, for example, six for a benzene ring (η6).

7.3

Pollutant adsorption mechanisms on clay minerals: An overview

General discussions of the adsorption mechanisms of organic molecules have sometimes been offered, be it in soil constituents (Gevao et al., 2000), or more specifically, in clay minerals (Mortland, 1970; Johnston, 1996; Lagaly, 2001; Cornejo et al., 2008; Boyd et al., 2011a; Thiebault et al., 2016). Unfortunately, there is no agreement on the terminology, so the same mechanisms are sometimes designated by different words.

Organic pollutant adsorption on clay minerals

201

Furthermore, authors may emphasise different distinctions, which they perceive as being more fundamental (Kulik, 2009). However, adsorption phenomena belong to the realm of chemistry, and may be treated in familiar chemical terms, with close parallels in homogeneous chemistry (both solution and solid state). Even the classical distinction between physisorption and chemisorption has more to do with the existence of separate scientific communities than with a clear-cut separation of adsorption mechanisms; and while in limiting cases, chemisorption may appear quite distinct from physisorption, in reality, there is a continuum of situations where one smoothly transforms into the other.

7.3.1 Electrostatic interaction and ion exchange If a molecule in solution has a net electric charge, whether intrinsic or due to acid–base equilibration, it will undergo an electrostatic interaction with a charged surface, such as that of clay minerals. One speaks of electrostatic adsorption when the energetics of the phenomenon can be described by modelling the molecule as a point charge. Electrochemists have long known how to treat this situation in the frame of the double layer model. The surface is modelled as a plane with uniform charge density, and the effect on the neighbouring electrolyte solution is to concentrate ions of the opposite charge while repelling those of the same charge. This effect is, of course, most pronounced in the immediate vicinity of the surface, and quickly decays with distance—over a few nanometres. This treatment may approximately describe the interaction of charged organic molecules with the external basal plane of a clay mineral particle, the surface charge being negative in this case. It is probably most appropriate for clay minerals with octahedral substitution such as montmorillonite (Mt), while in tetrahedrally substituted clay minerals (e.g. saponite or beidellite), the effect of substitutional charge is more localised, and it is an oversimplification to view it as spread out on the surface. Furthermore, the basal surface of clay mineral particles is not infinitely extended, and the energetics will be perturbed near the clay mineral edges; even when considering only electrostatic effects, the latter have a pH-dependent charge that will be positive at low pH. At any rate, in this situation the organic molecules would be excluded if negatively charged (this phenomenon is sometimes called negative adsorption), and if positively charged, they would be spatially distributed in exactly the same way as other compensating cations of the same charge (Na+, K+, …). Doubly charged ions, such as paraquat or diquat, would, however, be favoured over singly charged alkali metal ions (Valisko and Boda, 2007), and thus, are expected to be adsorbed preferentially in the case of pure electrostatic interaction. The situation is more complicated with molecules that bear several charges. In particular, if they are of opposite signs (zwitterion), an electric dipole results, and although the net charge is zero, the preferential orientation of the dipole may result in a favourable interaction with the charged surface (Chavez et al., 1996). Thus, electrostatic interaction is hardly ever a trivial matter. However, in a first approximation, if a molecule only interacts with the surface through electrostatic interactions, its spectroscopic properties will be unaffected with respect to those it has in solution.

202

Surface and Interface Chemistry of Clay Minerals

Ion exchange cannot be regarded as equivalent to electrostatic adsorption. It refers to replacement of compensating cations (in the case of clay minerals) inside a solid phase. It corresponds to a process that can be written as a chemical equilibrium, for example (the reader can refer to Chapter 4 of this book for more details on ion exchange): P + aq + Na +  clay mineral ¼ Na + aq + P +  clay mineral

(7.1)

Here, P+ is the cationic pollutant, and the subscript ‘aq’ (for ‘aqueous phase’) subsumes all the interactions between the ion and the solvent molecules in solution. In the same way, ‘Na+-clay mineral’ and ‘P+-clay mineral’ refer to all the interactions between the ion under consideration, and the clay mineral matrix, most of the ions being in the interlayer space. These interactions, of course, include electrostatic attraction, and in some cases, this is sufficient to explain differences in the adsorption behaviours of related molecules. For example, in a recent study of dyes adsorption by Mt, it was found that the cationic MB was adsorbed in quantities three times higher than the neutral Rhodamine B, while the anionic Orange II was not adsorbed—but the equally anionic sulforhodamine G was adsorbed (Fang et al., 2018). Other interactions than electrostatic are certainly present, even for the original Na+—which is generally coordinated by water molecules in the interlayer space. Thus, ion exchange must be viewed as a complex process involving several different kinds of interactions. Note that each molecule fixed in the clay mineral by ion exchange releases one compensating cation into the solution, which provides an easy way to quantify the phenomenon (e.g. Grauer et al., 1987b; Li et al., 2011a, 2011b; Wang et al., 2011; Fang et al., 2018). The possibility of ion exchange for a given molecule depends on its acid–base speciation, with acidic pH favouring the protonated € € forms, and thus the presence of a cationic charge (Ozcan and Ozcan, 2004). For example, diphenhydramine is only ion exchanged at pH < pKa, when its tertiary amine is protonated to the ammonium (Li et al., 2011a). Nalidixic acid is much more adsorbed on Mt at pH < pKa of its carboxylic acid group, when it is neutral, than at pH > pKa where the negative charge of the carboxylate causes a repulsion with the Mt layers. On neutral kaolinite, on the other hand, the difference is not pronounced (Wu et al., 2013a).

7.3.2 Covalent bonds and coordinative bonding From a physical point of view, of course covalent bonds are a consequence of the electrostatic interaction too, but electrons in molecules must be treated with quantum mechanics (Schr€ odinger’s equation or an approximation thereof ). This results in the archetypical chemical bond, namely two electrons (a bonding pair) between two atoms, that is schematised with a hyphen in organic structures—but it may also result in less familiar forms of bonding. It is perfectly conceivable that adsorption of a pollutant organic molecule involves the formation of localised chemical bonds, for example, with silanol or aluminol

Organic pollutant adsorption on clay minerals

203

groups at the edges of clay mineral layers. Indeed, in the case of glycine on smectite (the situation is relevant even though glycine is not a pollutant), it has been hypothesised that adsorption could involve the condensation of the carboxylic acid moiety with such groups on the clay mineral edge, to produce R–CO–O–Si and R–CO–O–Al ‘mixed anhydride’ bonds (Collins et al., 1988). However, further studies revealed that this reaction is only favourable if coupled with the opening of strained siloxane rings that exist on fumed silicas, but not on clay mineral edges (Rimola et al., 2013). More generally, the author is not aware of any instance where such a mechanism has been clearly demonstrated for organic pollutants on clay minerals. There is, however, one type of chemical bond that is highly relevant to pollutants/ clay mineral systems, namely, the coordinative bond with metal cations. In pristine clay minerals, compensating cations are most often alkali or alkaline earth ones, although Fe2+/3+ can also be present. The association of Lewis basic groups of ligand molecules with transition metal cations is the subject of coordination chemistry, and the splitting in energy of d orbitals under the effect of the ligand field is at the origin of interesting properties, including a specific energetic stabilisation called the crystal field stabilisation energy (CFSE). In the case of alkali and alkaline earth cations, the d orbitals are vacant, so that the CFSE is zero. However, coordination of various ligands does occur, although it is more difficult to study. For example, the calcium ion in the aqueous phase, denoted as Ca2 + aq , is actually a complex with 6 or 8 water molecules, [Ca(H2O)n]2+ (Bruzzi and Stace, 2017). In the clay mineral interlayer too, the original compensating cations are usually present as coordination complexes, as revealed, for example, by solid-state NMR—the coordination can vary according to the activity of water. If an organic molecule containing some ligand groups is introduced in the system, it will compete with water for coordination to the ion, and possibly form a coordination complex, or several different ones depending on the degree of hydration. The importance of this type of bonding was recognised early on by Tahoun and Mortland (1966) and by Jacobs et al. (1972). For example, 2,4-D is expected to form a monodentate complex with Ca2+ in a Ca+-Mt through its carboxylate group in the presence of water, but a bidentate one (thus occupying two coordination positions) in thoroughly dry conditions (Tunega et al., 2007). The process of displacing some of the original water ligands to allow coordination of the organic molecule is called ligand exchange. Here mention must be made of ‘ion–dipole interactions’ that are sometimes listed as a different type of interaction. In fact, they fall within the realm of coordinative bonding. In fact, the successful model of the ‘crystal field’ precisely rests on the idealisation of the coordinative bond as an ion–dipole interaction, because the ligand directs an electron-rich part of the molecule toward the central ion. The real coordinative bond is ionocovalent, and the partial covalence can be accounted for in more precise, molecular orbitals models. Is there, then, any need to maintain the term of ion–dipole interaction? The answer may be ‘no’ if one is talking to chemists, for whom ‘coordinative bonding’ will do perfectly; but ‘yes’ when talking to physicists, who may be more familiar with this view of the interaction.

204

Surface and Interface Chemistry of Clay Minerals

In some cases, it is assumed that organic molecules are coordinated to the compensating cation, and that the latter is also coordinated to the basal surface of the clay mineral, for example, to the siloxanes in the 6-ring of oxygen atoms. This is occasionally called an ‘ion bridge’. This has been proposed, for example, in the adsorption of methyl orange on Ca2+-palygorskite (Yang et al., 2018), or of alprazolam and diazepam in Mg2+-vermiculites (Palace Carvalho et al., 2014).

7.3.3 Hydrogen bonds, van der Waals interactions, and hydrophobic effects Here, these different effects are treated together because they correspond to weaker interactions than the preceding ones. It might then be thought that adsorption through these mechanisms counts as physisorption. That would be a mistake, because even if individual interactions are weak, several of them may be present between different groups of the adsorbed pollutant and the surface, or they may coexist with stronger interactions (see Section 7.3.4). Hydrogen bonding is a possibility to take into account in most cases (see Chapter 1 of this book for details). As previously underlined, many functional groups of common organic pollutants can act as H-bond donors and/or acceptors. They would certainly find partners on the clay mineral surface. Although the siloxanes of the basal surfaces are only weak H-bond acceptors, more diverse groups are found on the clay mineral edges. Silanols and aluminols can act as donors and as acceptors, and in other materials such as amorphous silicas, it is well established that ‘associated silanols’ are engaged at the same time in both types of interactions, each one reinforcing the other in a cooperative play (Rimola et al., 2013). Water molecules bound to the exposed octahedral cations on the edges are expected to be strong H-bond donors due to the electron-withdrawing consequences of their coordinative bonding to the cation. The same holds for hydration water attached to the compensating cations in the interlayer—it has long been known that the acidity of interlayer water is strongly enhanced (Mortland and Raman, 1968; Poinsignon et al., 1978). Finally, the structural OH groups of the octahedral sheets of TOT and TO clay minerals may be accessible through the ditrigonal cavities in the tetrahedral sheet, and thus play a role in H-bonding too. H-bond networks of more or less complexity may exist, a simple example being the formation of water bridges to metal ions or to silanols. Fusi et al. (1988) observed that fluazifop-p-butyl forms such a water-bridged link to the compensating cations in conditions of high relative humidity that can be reversibly transformed to direct coordinative bonding upon heating at moderate temperatures.

Organic pollutant adsorption on clay minerals

205

These two competing adsorption modes are sometimes called ‘outer sphere complexes’ and ‘inner sphere complexes’, respectively. Water bridges were also invoked to explain the interaction of atrazine with smectites (Laird, 1996; Xu et al., 2001), or of alachlor (Bosetto et al., 1993) and metolachlor (Bosetto et al., 1994) with Mt. ‘Hydrophobic interactions’ refer to the fact that molecules and other assemblies with little affinity to water prefer to stay together, and segregate from the water solution. This effect is well known in micelles formation and may play an important role too in the adsorption of molecules containing aromatic rings (which is a frequent feature of organic pollutants, as already mentioned). It is probably predominant in molecules such as the PAH that have no polar functional groups. On the side of the adsorbent, the siloxanes of the basal planes by themselves are considered to be hydrophobic, providing suitable partners for aromatic rings; but of course, in the case of smectites or vermiculites, there will also be compensating cations in the vicinity of these surfaces, and it may be unfavourable to displace them. A potential misunderstanding may arise here. There is nothing in physics that corresponds to ‘hydrophobic interactions’ as distinct from other interactions between molecules and chemical species. To be sure, hydrocarbon chains will be subject to the various types of weak interactions lumped together under the name of ‘van der Waals forces’ (see Chapter 1 of this book). They account for a slight attraction, of the order of only 1 or 2 kJmol1 for a pair of atoms. These figures rise, of course, as the van der Waals interactions are summed up over all atoms in the structure, but this does not seem enough to hold an assembly together. The solution to this difficulty is in taking into account the whole system involved in the adsorption process, as explained in the next paragraph.

7.3.4 Putting it all together: The importance of water A simple way of writing an adsorption reaction for a molecule A is Aaq +∗ 5A ∗

(7.2)

where * stands for a surface site, the special configuration of atoms that interacts with molecule A upon adsorption. Whether adsorption is favoured or not will depend on the sign of the standard free enthalpy of the adsorption reaction, ΔadsG°. In order to evaluate this parameter, all the contributions to the energy of the system have to be taken into account. Now contrary to what happens for adsorption from the gas phase, in the case of adsorption from aqueous solutions, all partners have significant interactions with water molecules—these interactions may be diverse, but at any rate, a more realistic depiction of ‘Aaq’ would be A(H2O)m. In the same way, the surfaces sites, whether occupied or not, are hydrated, so that instead of Eq. (7.2), one should write AðH2 OÞm + ∗ ðH2 OÞn 5A  ∗ ðH2 OÞp + ðp  m  nÞ ðH2 OÞ

(7.3)

in which a number of water molecules are liberated into the aqueous phase. If both molecule A and the surface are hydrophobic, these water molecules will be more stabilised when they are in the bulk solvent than they were initially when interacting with

206

Surface and Interface Chemistry of Clay Minerals

Fig. 7.2 Three modes of coadsorption of an organic molecule A with water.

A and *. This will suffice to make ΔadsG° negative, and the adsorption favourable. This is the main origin of the hydrophobic effect. Eq. (7.3) also has the merit of indicating that adsorbing molecules are in competition with pre-adsorbed water. They may, in fact, adsorb without establishing direct bonds to the surface, as in the case of water bridges mentioned previously. Schematically, the three situations could be considered for the coadsorption of an organic molecule with water (Fig. 7.2). Another general remark is that because of the complexity of some organic pollutants that often exhibit on the same molecule groups of different reactivity, and of the rich chemistry of clay mineral surfaces that also bear different types of sites, an organic molecule may establish several interactions with the surface at the same time. This is difficult to evidence experimentally, except in an indirect manner when the hypothesis of a single interaction fails to explain all the features of the adsorption data. However, the increasing use of molecular modelling suggests that multiple interactions indeed often occur. This raises the interesting possibility of a kind of interactional complementarity between an organic molecule and an inorganic surface, similar to the one that may exist with an enzyme: some molecules could be selectively adsorbed from the solution if they have a pattern of functional groups that matches the distribution of surface adsorption sites.

7.4

Tools of the trade: Experimental studies of organic pollutants adsorption

Several methods are available for the study of pollutants adsorption on clay minerals. Most of them are non-specific and can be used for other adsorbates too. One can distinguish between those providing macroscopic and molecular-level information.

7.4.1 Macroscopic characterisation 7.4.1.1 Isotherms, thermodynamics, and kinetics In the case of soils, the adsorption of pollutants is often studied through the use of breakthrough curves obtained from columns or lysimeters (dynamic adsorption tests) that are relevant for practical application in the field. The relatively high amounts of adsorbing materials that are needed for these studies generally prevent their use for pure clay minerals, although there are some exceptions (Cabrera-Lafaurie et al., 2015).

Organic pollutant adsorption on clay minerals

207

The adsorption of pollutants can be measured in batch experiments, even for small amounts of clay minerals. For low concentrations, radioactive labelling may be used (14C labelling, Nowara et al., 1997); otherwise, any method that can quantify the concentration in the aqueous phase before and after adsorption will allow determination of the adsorbed amounts from the solution depletion. This can often be done by UV–visible spectroscopy. In this way, ‘adsorption capacities’ expressed, for example, in mg/g, can be obtained, and they are listed, for example, for dyes on various clay minerals (Ngulube et al., 2017). They can be fairly high, up to 500 mg/g clay mineral, but in general they vary with concentration in the aqueous phase, pH, and so forth. At times, ‘negative adsorption’ is observed, that is, the pollutant concentration in the solution apparently increases upon contact with the clay mineral. This is the case, for example, for aldicarb on Mt, while positive adsorption occurred in illite and kaolinite (Supak et al., 1977), or to a smaller extent, atrazine on smectites (Barriuso et al., 1994). It only makes sense if one considers that the clay mineral retains an amount of ‘bound water’ that will not be probed by ordinary analytical techniques, and where concentration conditions may differ from the bulk. If a molecule is excluded from the bound water layers due to unfavourable interactions, its concentration will necessarily increase in the bulk. An empirical parameter that is often tabulated for various pesticides in soils (Clausen et al., 2001; Delle Site, 2001; Drillia et al., 2005) is the distribution coefficient Kd. It simply consists in the ratio between the amount of pesticide fixed by the soil (which may be given in mole per gram, mol g1) and the concentration remaining in solution at equilibrium (expressed in mol L1): in this case the units of Kd will be L g1. Alternatively the adsorbed amount may be expressed as a surface concentration (mol m2), taking into account the surface area of the soil (S in m2 g1), the units of Kd will then be L m2. At any rate, this corresponds to a simple picture of adsorption as an exchange between two phases, the aqueous solution (aq) and the solid soil:

Aaq

Asoil

ð7:4Þ

where Kd is then the selectivity constant of this exchange. As such, this model does not include any limit on the adsorption capacity of the soil, which is not realistic, as shown in the following paragraph. Thus, these values should be considered more as practical guidelines for soil amendment than as fundamental properties. They will depend on the intensive thermodynamic parameters of the aqueous solution, such as the pH (Weber et al., 2004). They will be concentration-independent only in the range where the adsorption isotherm is linear (e.g. Langmuir at low concentrations, see the following). Adsorption isotherms can easily be established by plotting the adsorbed amounts (Γeq, if expressed as surface concentrations) as a function of the concentration in the aqueous phase Ceq (after the adsorption equilibrium is established). They can be modelled using one of a few classical models of physical chemistry. For a basic presentation of these models, see Chapter 8 of this book. The most commonly used one for organic pollutants is the Freundlich isotherm:

208

Surface and Interface Chemistry of Clay Minerals 1

Γ eq ¼ Kf Ceq n

(7.5)

which depends on the two parameters Kf and n. One can check if the data fit this model by representing them on a logarithmic scale. The Langmuir model is also commonly tested: Γeq ¼ Γm

bCeq 1 + bCeq

(7.6)

Like the Freundlich model, it also depends on two parameters, but their interpretation is more intuitive: b is the equilibrium constant of the adsorption reaction and Γm is a saturation coverage (or a saturation quantity if the adsorbed amounts are expressed per gram instead of per unit surface). The same criticisms can be made as for the application of these models to proteins. The Freundlich isotherm has a heuristic value in that it allows one to summarise the data obtained with a single, simple equation. Kf is related to the adsorbed amounts, and therefore comparing the Kf values of different clay minerals for the same pollutant, or of different pollutants on the same clay mineral, may give a general impression of the relative adsorption capacities. On the other hand, the physical bases for the Freundlich model are not clear, apart from the fact that the adsorption energy is not constant with the amount adsorbed, in opposition to the Langmuir model. This could be due to the existence of different adsorption sites, to interactions between neighbouring adsorbed organic molecules, or to multilayer adsorption (in this case though, the BET model would probably be more appropriate). The very fact that so many adsorption data for various pollutants may be fitted by a Freundlich isotherm suggests that this does not tell us anything fundamental about the adsorption process. In some instances, it has been observed that the Langmuir isotherm is a better fit than the Freundlich one, for example, for ciprofloxacin on Mt, rectorite, and illite (Wang et al., 2011), where the adsorption mechanism seemed to be ion exchange, and the saturation quantity indeed corresponded well to the CEC. Similar cases are pyrene on sepiolite (Sabah and Ouki, 2017), diphenhydramine on Mt (Li et al., 2011a), MB on Mt (G€ urses et al., 2004), levofloxacine on Fe-PILC (Liu et al., 2015b), glyphosate on Mt (Khoury et al., 2010), and to a lesser extent, tramadol and doxepin on Mt (Thiebault et al., 2015). In their comparative study of dyes adsorption on Mt, Fang et al. (2018) found that the Langmuir model was more appropriate for the cationic MB, which was ion exchanged; but for the zwitterion Rhodamine B, the Freundlich model gave a better fit. On palygorskite, Yang et al. (2018) found a better fit with Langmuir for cationic dyes and, unexpectedly, also for anionic ones. Other studies on related systems could not discriminate between the two models, for example, for methyl violet adsorption on halloysite (Liu et al., 2011a), bisphenol-A on palygorskite-Mt formulations (Berhane et al., 2016), prometryn on halloysite (Grabka et al., 2017), or crystal violet on Mt (Sarma et al., 2016). The Freundlich model was found to be better for gemfibrozil, mefenamic acid, and naproxen on vermiculite (Dordio et al., 2017),

Organic pollutant adsorption on clay minerals

209

naphthalene on bentonite (Kaya et al., 2013), and for Congo Red on bentonite (Vimonses et al., 2009). Initial Langmuir adsorption followed by further adsorption (possibly due to the inception of a multilayer) was invoked for nonionic pesticide on kaolinite and Mt (Torrents and Jayasundera, 1997). The general impression from these studies is that the Langmuir model fits well in situations dominated by ion exchange, with a saturation adsorption corresponding to the CEC; a better fit of the Freundlich model, and especially of the BET one (appropriate for multilayers), would indicate significant adsorption on the edges (Fang et al., 2018). A more refined, two-step Langmuir model was applied to understand glyphosate adsorption on Mt (Khoury et al., 2010), which seemed to imply both adsorption on the edges and intercalation. Of course, deconvoluting two adsorption mechanisms that occur simultaneously requires high precision data. The reader must be cautioned that fit does not prove a mechanism, and that all studies mentioned here must be considered as empirical. When it applies, the Langmuir model gives access to the adsorption equilibrium constant b, and from there to ΔadsG° (ΔadsG° ¼  RT ln b). Because ΔadsG° ¼ ΔadsH°  TΔadsS°, a study of adsorption at different temperatures should allow one to separately estimate the enthalpic (ΔadsH°) and entropic (ΔadsS°) contributions to the adsorption reaction. This is not done systematically. For MB adsorbed on a Turkish clay mineral, ΔadsH° and ΔadsS° were 8.0 and +25.5 kJ1°mol1, respectively, meaning that enthalpic and entropic effects were both favourable and contributed equally to the adsorption at RT (G€urses et al., 2004). For crystal violet adsorbed on several Mt, ΔadsH° ranged from 18.1 to 21.6 kJ mol1, and ΔadsS° from +49 to +60 kJ1 mol1 (Sarma et al., 2016). The very idea of an adsorption isotherm supposes that adsorption is a reversible phenomenon that can be described by an equilibrium. Some systems show partial irreversibility, manifested by a hysteresis between adsorption and desorption branches; a cdes  cads , where cads and cdes hysteresis index was proposed, and defined as HI ¼ cdes refer to the concentration of adsorbed pollutant measured in the adsorption branch and in the desorption branch, respectively, for the same concentration in the solution. The apparent irreversibility may be due to the fact that the desorption process has kinetics that are too slow to be measured in practice. In the case of carbamazepine adsorbed on smectites, hysteresis was attributed to the inception of π–π interactions between the aromatic moieties (Zhang et al., 2010). Such phenomena are, of course, important for controlled release formulations. For atrazine adsorbed on smectites, hysteresis was reported to be positively correlated to the layer charge (Barriuso et al., 1994). It was also observed for alachlor, metolachlor, and linuron adsorbed on Na+-Mt and kaolinite (Torrents and Jayasundera, 1997), or bisphenol A and ciprofloxacin adsorbed on palygorskite-Mt (Berhane et al., 2016). The kinetics of adsorption are also often evaluated, and they are fitted to either firstorder (or Lagergren, i.e. pseudo-first order) or second-order models. As for the adsorption isotherms, a basic presentation of adsorption kinetics models is found in Chapter 8 of this book. A review of second-order kinetics for pollutants adsorption may be found in Ho and McKay (1999), but it does not deal primarily with pure clay minerals. The Elovich model is not often tested, and when it is, it does not seem particularly

210

Surface and Interface Chemistry of Clay Minerals

appropriate (Kaya et al., 2013; Fil et al., 2014). While the pseudo second-order model often gives a good fit to the data, once again this has a mostly heuristic value. The main conclusion regarding adsorption mechanism is that diffusion is not rate limiting. Adsorption is generally completed in, at most, a few hours; although in high layer charge clay minerals such as vermiculite, very slow adsorption kinetics have been reported, up to 10 days for completion (Rytwo et al., 2009). The knowledge of at least an order of magnitude for the kinetics is important for practical applications, and a prerequisite for equilibrium adsorption measurements such as those needed to obtain isotherms. Adsorption kinetics, like adsorption isotherms, are generally evaluated by solution depletion in batch experiments; however, for an IR-based method, allowing to follow adsorption kinetics in situ.

7.4.1.2 Thermal analysis Thermal analysis methods include thermogravimetric analysis (TGA) and differential thermoanalysis (DTA). TGA simply records the sample mass as a function of time and allows one to evidence those thermal transformations that are accompanied by mass loss, such as desorption or calcination for clay mineral–organic systems. These events are best revealed by plotting the DTG (derivative of the TG signal). TGA may be coupled with EGA (evolved gas analysis), by mass spectrometry, or sometimes IR. DTA measures thermal effects, whether endo- or exothermic, and may detect phenomena that are not accompanied by mass changes, such as phase transitions. TGA may allow one to estimate the total amount of pesticide bound to a clay mineral, if carried out under an oxidising atmosphere where the organic matter is burned out (Ibarguren et al., 2014; Wanyika, 2014). Under an inert atmosphere too, it usually shows peaks attributable to the adsorbed organic molecule (Tomic et al., 2016), but in that case, part of the organic matter often pyrolyses transforming to inert carbon that remain adsorbed on the clay mineral particles, so that quantification is more difficult. By comparison with the TG of the bulk xenobiotic, it allows one to also observe stabilisation, or conversely, catalysed thermal decomposition. Thus, several PAH were observed to desorb from bentonite, but at temperatures significantly above their bulk boiling point (up to 500°C), indicating that they interacted strongly with the clay mineral surface (Biache et al., 2015). At these temperatures, part of the PAH decomposed to lower mass aromatics instead of desorbing intact. Because the elimination of physisorbed water occurs in a separate temperature range from that of organic molecules decomposition, TG also permits, if conducted in carefully controlled conditions, one to observe the displacement of bound water upon adsorption of the organic molecules (Wanyika, 2014), revealing competition between them. DTA has been used to discriminate between free and Mt-bound forms of alachlor (Nasser et al., 1997), and evidences the competitive replacement of water with alachlor. The melting peak of promethazine was suppressed when it was deposited on Mt (Ambrogi et al., 2014), confirming the absence of bulk crystals (in contrast, a physical mixture of these and the Mt, with the same global content, did show the melting transition).

Organic pollutant adsorption on clay minerals

211

7.4.1.3 Electrophoretic mobility Particles in a dispersion will migrate in a potential difference, and the measurement of this effect gives access to a quantity called the zeta potential. This depends on the charge of the particles—not exactly the surface charge, but that of the particle accompanied by a few layers of bound water. Zeta potential measurements are often used to characterise the surface properties of dispersed clay mineral nanoparticles as a function of pH. Amphoteric nanoparticles, in the absence of specific adsorption, are positively charged at low pH and negatively charged at high pH; at the isoelectric point (IEP), the net charge is zero. Smectites do not have an IEP because their charge is mostly substitutional in origin, and the zeta potential is negative, irrespective of the pH. In principle, when a charged species is adsorbed on a smectite, the zeta potential will be modified accordingly, and this technique might therefore provide a means to follow adsorption in situ. The data are rather scarce; ofloxacin-loaded activated halloysite had positive zeta potentials, indicating that the surface of halloysite nanotubes is positively charged due to the adsorption of ofloxacin (Wang et al., 2013). Zeta potential measurements are also reported in Pessagno et al. (2008), Maqueda et al. (2013), and Ibarguren et al. (2014), but not much commented upon.

7.4.2 Molecular level characterisation 7.4.2.1 X-ray diffraction (XRD) XRD can indicate if a fraction of the pollutants has precipitated as bulk crystallites, as opposed to being molecularly adsorbed. This situation is perhaps unlikely to be met in equilibrium adsorption studies, but it can certainly occur in attempts to prepare clay mineral–xenobiotic composites for controlled release. Thus, Fraile et al. (2016) observed that crystallites of dexamethasone form upon attempts to deposit high loadings of the corresponding molecule on Laponite for a controlled release drug therapy application. Crystallites were also observed in controlled release formulations of amitrole with methoxy-modified halloysite and kaolinite (Tan et al., 2015), and their presence was not necessarily detrimental to release efficiency. In such cases, it may be surmised that the clay mineral has a saturation loading for the considered xenobiotic. Forcing the deposition of higher loadings in the solid phase by evaporation of the clay mineral–solvent system results in the excess part precipitating more or less independently from the clay mineral surface. Apart from this, the main information that can be drawn from the X-ray diffractograms of pollutants–clay minerals systems is the d001 value, which is the sum of the clay mineral layer thickness (assumed to be invariable) and the interlayer thickness. In smectites, the interlayer spaces can swell, and this permits intercalation of more or less bulky molecules, which is revealed by an increase of d001. However, the data may be ambiguous because, depending on the water activity, smectites can exist in different hydration states: one can observe an anhydrous form, a 1-layer form and a 2-layer form, with increasing d001 values. Because the XRD reflections of clay

212

Surface and Interface Chemistry of Clay Minerals

minerals are rather broad, it may be difficult to decide if a given sample has a d001 significantly different from that of the hydrated forms. In this case, the best procedure is to determine if there is a collapse of the layers upon dehydration (Grauer et al., 1987b). If d001 stays high at low water activities, it probably means that large organic molecules in the interlayer space oppose collapse. With big molecules, though, the swelling may be large enough that no ambiguity remains, as with enrofloxacin, which ˚ (Nowara et al., 1997), which cannot be swells Mt up to d001 values of more than 19 A ˚ when due to water alone. Diphenhydramine caused the swelling of Mt to 17 A adsorbed in high amounts, and because this reflection was not observed for a physical mixture in the same conditions, this was attributed to intercalation (Li et al., 2011a). There are many examples of xenobiotic intercalation, and unsurprisingly they include a lot of positively charged molecules, such as MB (Hang and Brindley, 1970), which forms the basis of a popular method of surface area measurement for clay minerals in aqueous dispersions. They also include molecules that can be protonated to the cationic form, as already mentioned herein; other examples include benzimidazole in Mt (Lombardi et al., 2006), and chloroanilines in Mt (Angioi et al., 2005). The same molecule can give different basal spacings due to different stacking modes in the interlayer space, which is the case for MB in Na+-smectites (Hang and Brindley, 1970). It has been proposed that the entrance of picloram into the Mt interlayer space may occur in two different arrangements, perpendicular and planar, depending on the xenobiotic concentration (Marco-Brown et al., 2015). Donepezil intercalated forming double layers in Mt, but only a single layer in Laponite and saponite (Park et al., 2008). Alachlor expanded the interlayer spaces of Mt, but not those of pillared Mt (Nasser et al., 1997) where the pillars are inorganic species that may bind strongly enough to the layers to prevent them from moving apart. In the same way, ciprofloxacin swelled Mt and rectorite, but not illite (Wang et al., 2011). There are many studies that compare the adsorption of substances on Mt and on kaolinites, and in a lot of cases Mt swells while kaolinite does not (see Table 7.1 for examples).

Table 7.1 Studies reporting the intercalation of pollutants in montmorillonite, with comparison to kaolinite References

Pollutants studied

Torrents and Jayasundera (1997)

Alachlor, metolachlor, linuron

Gianotti et al. (2008)

2,4,6-TCA, chlorophenol

Angioi et al. (2005) and Gianotti et al. (2008)

Chloroanilines

Oudou and Hansen (2002)

Pyrethroids

Shareef et al. (2006)

Endocrinedisruptors

Styszko et al. (2015)

Bisphenol A, triclosan

Gimsing and Borggaard (2002)

Glyphosate

Organic pollutant adsorption on clay minerals

213

Even for smectites, swelling will depend on the original compensating cation, being more limited and more difficult with Ca2+-clay minerals than with Na+-clay minerals (Hang and Brindley, 1970; Boyd et al., 2011a). Swelling was also reported in conditions when the speciation of the xenobiotic in solution is supposed to favour a neutral form. First, it must be remarked that the local speciation inside the clay mineral layers—and also in the vicinity of the external surfaces—may be altered with respect to that in solution. Consider a molecule involved in an acid–base equilibrium: Aaq + H + aq ¼ AH + aq

(7.7)

Suppose further that the conditions in solution favour the unprotonated form (which means that pH > pKa of the couple). If the ion exchange of species AH+ into the clay mineral (along reaction (7.8) below) is favoured, that is, if its interactions with the clay mineral layers are more favourable than those of the cation in the pristine clay mineral, then reaction (7.7) will be drawn to the right according to Le Ch^atelier’s principle, and the global process (7.9) may be favourable. AH + aq + Na +  clay mineral ¼ Na + aq + AH +  clay mineral

(7.8)

Aaq + H + aq + Na +  clay mineral ¼ Na + aq + AH +  clay mineral

(7.9)

In that case, the pH will increase upon ion exchange, because H+ will be removed from the solution (or alternatively, OH will be formed because the H+ come from the dissociation of water). Thus, a complete study of adsorption should include measuring the release of the pristine compensating cations (to check if cation exchange occurs), and measuring the pH change (to obtain information on the comparative interactions of both cations with the clay mineral matrix). However, intercalation is not synonymous with ion exchange. The driving force for the migration of molecules into the interlayer space may be another one, such as a coordinative interaction with the compensating cations (Damonte et al., 2007), or the establishment of hydrophobic interactions with the basal planes in the interlayer space. For example, DNC molecules were intercalated in Mt interlayer spaces with their aromatic groups lying flat between the two layers (Sheng et al., 2002). ˚ , even under In this case, the d001 of the intercalate remained at the low value of 12 A conditions of high humidity, meaning that the intercalated species opposed not only collapse, but swelling too—in order to stay able to interact with both layers simultaneously. In the same study, it was also observed that exchange of the Mt with weakly hydrated cations (K+, Cs+) resulted in higher DNC adsorption than when starting with more hydrophilic cations (Na+, Ca2+), meaning that coordination of the –NO2 groups to the cations, by ligand exchange with the originally present H2O ligands, contributed to the driving force for intercalation. Not only neutral molecules such as DNC or 2,4,6-TCA and 4-chlorophenol (Gianotti et al., 2008) but also negatively charged species can be intercalated by a ligand exchange mechanism, and this has been observed with glyphosate at least at high concentrations

214

Surface and Interface Chemistry of Clay Minerals

(Damonte et al., 2007). Indeed glyphosate, a polyacid that may develop up to three negative charges, intercalated in Mt in conditions where its global charge was 1 or 2 (Khoury et al., 2010). However, locally, it also carried one positive charge on the amine group, so that this may again be a case of cooperative interactions—the negative phosphate group interacting with the cation through a coordinative bond, while the NH2 +  group would interact with the negatively charged clay mineral layers.

7.4.2.2 Transmission electron microscopy (TEM) TEM may be used as a complementary technique to XRD to observe swelling— directly on micrographs when stacked layers in smectite-based nanocomposites may be very clearly visible. A more original application is to use the lateral resolution of microanalysis (SEM-EDS, scanning electron microscopy with energy-dispersive X-ray spectroscopy) in order to differentiate adsorption on the edges from adsorption on the planar surfaces (Joseph-Ezra et al., 2014). For PAH adsorbed on Cu2+-Mt, measurement of the organic carbon contents indicated that adsorbed molecules concentrated at the outermost part of the clay mineral particles, which is compatible with adsorption on edge sites.

7.4.2.3 UV–visible absorption and fluorescence UV–visible spectroscopy may, of course, be used to quantify pollutants in solution, but in this chapter the concern is its use for characterisation of the adsorbed pollutants. Dyes are, of course, good candidates for detection in the UV–visible, and visually perceptible colour changes may occur upon contact with clay mineral dispersions. Dyes may also be fluorescent, allowing a study in the adsorption, as well as the emission mode. Sometimes, the aggregation of dye molecules can give rise to new absorption bands, a phenomenon called metachromasy. In fact, for MB, several bands are observed corresponding to different association states of the dye molecules—monomers, dimers, and agglomerates. On a series of Mt whose charge had been modified by the Hofmann– Klemen effect, Bujda´k and Komadel (1997) found that low layer charges favoured the monomers, while high layer charges favoured agglomerates—even though they worked in conditions of low MB loading where the loadings for all smectites were the same. They could also evidence a slow reorganisation of the adsorbed MB (over a few days) that could not have been observed otherwise because the total adsorbed amount did not change. On the fundamental level, metachromasy is due to the interaction of the π electron systems of two or more dye molecules. It has been observed that the π electrons of a single adsorbed dye molecule can also establish similar interaction, probably involving the oxygen atoms of the basal plane, and undergo a blue shift; this was shown by for several dyes, including MB (Yariv and Lurie, 1971), acridine orange (Cohen and Yariv, 1984), and pyronin (Grauer et al., 1987a). Rhodamine B, on the other hand, showed a red shift instead on adsorption to Mt, perhaps because steric hindrance prevented the interaction of π electrons with basal planes. In these studies, the effect of molecule–molecule interactions is difficult to disentangle from that of molecule–clay mineral interactions.

Organic pollutant adsorption on clay minerals

215

A new fluorescence polarisation method was applied to evaluate the preferential orientation of rhodamine 6G adsorbed into Laponite (Lo´pez Arbeloa and Martı´nez, 2006). The fluorescence dichroic ratio was used to evaluate the orientation of the fluorescent species adsorbed in the clay mineral films. It was found that the plane of the polyaromatic ring was tilted 30 degrees from the basal plane, and a minor amount of coplanar rhodamine dimers was also observed.

7.4.2.4 Vibrational spectroscopy Vibrational spectroscopy is one of the main techniques of identification and characterisation of organic molecules. It has been available for a long time, and as a consequence, it was applied to organic molecules on clay minerals since the first studies of these systems. Clay minerals strongly absorb IR in the 1200–900 cm1 region due to lattice vibrations, and to a smaller extent, in the 3800–3200 cm1 region due to OH stretching. They have a window of transparency between 3000 and 1300 cm1 (except for the δHOH of adsorbed H2O at about 1630 cm1). This is particularly fortunate because this region includes several diagnostic bands of organic pollutants, such as the amide I and II, the stretching (νC]O) of carboxylic acid groups, the asymmetric and symmetric stretchings (νas and νs) of carboxylates, and the bending vibrations (δas and δs) of amines and ammoniums. Other, less common functional groups may also be observed, such as the P]S of some organophosphorous compounds (Sa´nchez Camazano and Sa´nchez Martı´n, 1980) or the N]O in nitro compounds (Johnston et al., 2001). The near IR portion of the spectrum potentially contains useful information too, in the form of overtone and combination bands, but it is less often explored, an exception being (Tomic et al., 2012). IR in the transmission mode can be used to verify the presence of adsorbed organic molecules in the solid, and to some extent, to quantify them. Absolute quantification is almost impossible because the intensities of the IR bands depend on their extinction coefficients, which are difficult to estimate a priori. But relative quantification is possible, for example, in DNB adsorption on smectites; Johnston et al. (2001) found that the intensity of the N]O stretching was proportional to the amount of adsorbed DNB, independently derived by HPLC. In the same way, in a study of dibenzop-dioxin on smectites Rana et al. (2009) found that the intensity of dioxin bands correlated well with the adsorbed amount from HPLC—in addition, the relaxation of selection rules for the dioxin molecule allowed one to garner information on its interaction with Cs+ ions. Relative quantification is also possible at the solid–solution interface by using a different experimental setting (attenuated total reflectance or ATR mode). In this way, the intensities of the aromatic and COO bending bands were used to follow in situ the kinetics of picloram adsorption on Mt (Marco-Brown et al., 2014). Most often, the bands of the adsorbing substance can be easily recognised, meaning that adsorption occurs without degradation. There may still be significant band shifts or other modifications; however, that provide useful information on the clay mineral/ organic molecule interaction. For example, IR easily distinguishes carboxylic acid from carboxylates (Nowara et al., 1997) and this allows us to know the predominant species of the acid–base couple in the adsorbed state. The protonation state of amines

216

Surface and Interface Chemistry of Clay Minerals

and other nitrogen containing groups is also accessible. For example, N-heterocycles such as benzimidazole and thiabendazole in Mt showed a band at 1624 cm1, indicating that the nitrogen was protonated to the cationic form (other evidence suggested ion exchange with intercalation into the interlayer spaces). The position of IR bands is sensitive to more subtle effects than protonation/ deprotonation. Coordination to cations has a strong influence on the vibrations of the coordinated moieties: coordinative bonds of C]O groups usually give rise to shifts of their stretching vibrations higher than 40–50 cm1, while smaller shifts are assigned to H-bonding effects (Fusi et al., 1988). In the heterocycle nalidixic acid, ring vibrations involving the C]N1 are observed at 1475 and 1521 cm1 upon adsorption, they shifted by +13 and +8 cm1, respectively, which was attributed to coordination to the cation through the N1 atom (Wu et al., 2013a). Nasser et al. (1997) interpreted IR band shifts as indicative of H-bonding of both the amine and carbonyl moieties of alachlor, supposedly with water molecules in the interlayer. The vibrations of the clay mineral lattice itself may be affected by adsorption. Rytwo et al. (2002) observed the δXOH bending mode and the νOH stretching in sepiolite and followed them upon the addition of cationic xenobiotics. These authors observed band shifts and broadenings with MB, but not with Diquator Paraquat, and deduced that only the first compound interacted with the silanol groups (through H-bonding). More precise techniques can give information on the orientation of the adsorbed molecule with respect to the clay mineral matrix. Sheng et al. (2001) prepared self-supported oriented films of DNP containing smectite (SWy-2), and measured the dependence of the in-plane and out-of-plane NO2 vibrations (νsym and ωoop, respectively) for incident beam angles of 0 and 45 degrees. The intensity of the first one decreased at 45 degrees, while that of the second one increased, indicating that the molecules were oriented parallel to the clay mineral layers. This technique may be compared with the fluorescence polarisation method previously mentioned. In another study by Rana et al. (2009), polarised ATR revealed that dioxin molecules were tilted, this time by 65 degrees, with respect to the basal planes. Of course, IR spectroscopy can detect the appearance of new molecules in conditions that lead to adsorbed pesticide degradation. Thus, the slow adsorption of crystal violet molecules on vermiculite led to the appearance of methylene groups that are not present in the original dye (Rytwo et al., 2009), pointing at degradation of the molecule in soft conditions.

7.4.2.5 X-ray photoelectron spectroscopy (XPS) XPS is a spectroscopic technique able to provide information on the atomic composition of the topmost surface layers of a sample. While its capabilities for getting chemical information remain limited, it can still be useful in conjunction with other techniques, beyond confirming that the stoichiometric ratios in the surface organic matter (e.g. S/C or N/C) are compatible with those of the adsorbed pollutant molecule. The position of the N 1s peak is sensitive to the degree of protonation of the nitrogen

Organic pollutant adsorption on clay minerals

217

atom, and therefore XPS can discriminate between the acidic and basic forms of a couple involving these atoms. It has been applied, for this purpose, to thiabendazole and benzimidazole on Mt (heterocyclic N, Lombardi et al., 2006) and glyphosate on Mt (secondary amine, Khoury et al. 2010). A more complex situation was observed with picloram that has a primary amine as well as a heterocyclic nitrogen (pyridine ring), both susceptible of protonation. Marco-Brown et al. (2015) argued that through a quantitative comparison of the spectra of picloram adsorbed on Mt and a physical mixture, they could discriminate the contributions of the two species, and evidence coordinative bonding of the heterocyclic N, as well as protonation of the amine. When interpreting XPS spectra, however, one must keep in mind that they are obtained in conditions of ultrahigh vacuum, where the speciation may be different from that in solution (for example, glycine is a zwitterion in solution, but a neutral molecule under vacuum).

7.4.2.6 Solid-state nuclear magnetic resonance (NMR) Solid-state NMR has a huge potential to characterise the interactions of organic molecules with inorganic matrices (Bonhomme et al., 2014). Yet, surprisingly, it is used much less often in the field of organic pollutants adsorption than, for example, in that of clay mineral based nanocomposites. There are, of course, some exceptions. Fraile et al. (2016) recorded the NMR spectra of dexamethasone in solution, in bulk crystals, and adsorbed on Laponite. They observed significant shifts in the signals of some carbons upon adsorption and interpreted them as due to the interaction of the C3 carbonyl and C11 alcohol groups (see numbering in scheme of Appendix) with electronwithdrawing centres on the clay mineral surface, either through coordination to the compensating cations or through H-bonding. Advanced solid-state NMR techniques were applied to pollutants on clay mineral systems by Maciel’s group. A variable temperature study of the deuterium NMR line shapes of D-enriched benzene gave access to rich information on the local motions and mobility of this pollutant in Ca2+-Mt (Xiong and Maciel, 1999). It was concluded that at low temperature (75°C), benzene is able to coordinate to the interlayer Ca2+ ions through its π electrons (π–cation complex). As the temperature increased, more and more benzene molecules gained freedom from this coordinative bonding, and at room temperature, about one half of the benzene molecules are desorbed from the cation and reorient freely. A previous study of benzene adsorption in Cu2+-Mt had involved solid-state 13 C NMR (Hinedi et al., 1993). Only poor quality spectra were obtained due to paramagnetic interaction with Cu2+ centres, but signals with chemical shifts in the aliphatic region were obviously present, showing that benzene was chemically unstable in this environment. Indeed, NMR has been applied most often to situations where the adsorbed pollutants underwent some type of degradation (see Section 7.5). Thus, 31 P NMR was used to explore the decomposition of chlorpyrifos to inorganic phosphate (Seger and Maciel, 2006); 13C solid-state NMR played a role in identifying the reaction products between two pollutants in Mt, namely CCl4 and benzene (Tao and Maciel, 1998), or the photodegradation pathways of trichloroethylene in Ca2+-Mt (Tao et al., 1999).

218

Surface and Interface Chemistry of Clay Minerals

7.4.3 Molecular modelling Applications of molecular modelling to adsorption phenomena are treated in detail in Chapter 3 of this book. With the huge increase of computing power, molecular modelling studies of pollutant/clay mineral systems are becoming increasingly common. They include density functional theory (DFT), molecular mechanics, and hybrid methods that sometimes allow one to model systems containing many atoms. Occasionally, molecular dynamics (MD) simulations are also included. Because rather weak interactions such van der Waals and H-bonds often play a determining role in adsorption, it is important to use dispersion-corrected computational methods (Belzunces et al., 2017). Molecular modelling should be particularly appropriate to test cooperative adsorption scenarios where the organic molecule establishes several different interactions with the surface, especially if already available experimental data constrain the possibilities—otherwise, the adsorption scenarios might simply be too numerous to test all of them. Chatterjee et al. (2002) studied the bonding of dioxins to Mt and beidellite in the frame of the hard–soft acid–base (HSAB) theory. They defined a local softness as a reactivity index and used it to predict the interaction of organic molecules with the structural OH in the octahedral layer as a function of layer substitution (the molecules acted as H-bond acceptors through their chlorine atoms). They predicted beidellite to be the best adsorbent, through formation of a H-bond between a chlorine and the octahedral OH. The weakness of this approach is that the clay mineral was represented by a small cluster that did not include edge sites. Dioxin adsorption on smectites with varying compensating cations was studied by MD (Liu et al., 2012), and the experimentally observed adsorption trends were reproduced by considering a complexation of the cations by the oxygen in the heterocycle. In a study of triazine herbicides in Mt and saponite, Aggarwal et al. (2006) found a favourable interaction with the aromatic cycles interacting with both clay mineral layers, and the N and Cl atoms of the organic molecule simultaneously coordinated to K+. At the same time, the alkyl groups were well positioned to interact with ‘hydrophobic microsites’ on smectite surfaces (the siloxane planes). Bonding of the pollutant molecule to the clay mineral was thus a highly cooperative phenomenon. This study combined MD with XRD data indicating that dioxin molecules were intercalated in the interlayer space, and adsorption isotherms that showed inter alia a much higher loading on Cs+-exchanged clay minerals. The latter observation is consistent with a ligand-exchange mechanism because water is easier to displace from bulky, chaotropic cations. Tunega et al. (2007) studied the adsorption of 2,4-D on Ca2-Mt using MD. These authors compared adsorption on a dry external surface, on a hydrated one, and upon intercalation. Coordinative binding of the COO moiety to the Ca2+ compensating cation is favourable in the first two cases: it is bidentate in the absence of water, monodentate in its presence. This study paid attention to the H-bonding of water, with the adsorbed organic molecule, the clay mineral surface groups, and among H2O molecules; multiple H-bonding (H-bonding lattice) made an important energetic contribution to the adsorption. However, Ca2+ could not be considered as forming ‘cation bridges’ with the surface, as previously proposed. Meleshyn and Tunega (2011), using a periodic model of the basal plane, found that phenanthrene as a model PAH could

Organic pollutant adsorption on clay minerals

219

favourably interact with a compensating Na+ cation through cation–π complexation; however, when present, water outcompeted phenanthrene, suggesting that PAH can only be significantly retained by clay minerals in dry soils. Rather different conclusions were drawn by Hou et al. (2015) in a study with periodic DFT including explicit water molecules: for aromatics (toluene and anisole) adsorption on Mt, cation–π interactions played an important role, in subtle cooperation with H-bonds to the siloxanes, of the organic molecule itself, and also of the competing water molecules. Overall, and somewhat counter intuitively, the presence of water reinforced the adsorption of the aromatic molecules. Shapley et al. (2013) using DFT and force fields, tried to distinguish between ‘inner sphere’ and ‘outer sphere’ complexation of PCDD molecules on pyrophyllite and Mt, that is, direct coordination or coordination through a water bridge. The free energy of adsorption did not depend much on the choice of the compensating cations. Diffusion coefficients were also evaluated: on pyrophyllite, PCDD was predicted to be more favourably adsorbed, but also more mobile on the basal planes than on Mt, corresponding to a picture where it efficiently displaces water from the first adsorption layer, while on Mt it is presumably ‘pinned down’ by localised charges. It must be underlined that this study included a modelling of the edge sites and not only of the siloxane planes. Palace Carvalho et al. (2014) studied the interaction of benzodiazepines on Mg2+vermiculite (periodic DFT, without edge sites, limited number of explicit water molecules). These authors found that the energetically most favourable adsorption modes were through direct coordination to a nitrogen atom from the heterocyclic ring, and through the formation of a H-bond with a water molecule that was itself bound to the surface (this is different from what was called ‘water bridges’ previously). Liu et al. (2015a) noticed that traditional MD methods were too expensive computationally to accurately model transport processes in clay minerals. These authors applied enhanced MD methods that sample the conformational space faster for the adsorption of PCB (polychlorinated biphenyls) and PAH on Mt. They found that PCB were most strongly adsorbed in the interlayer spaces, if their aromatic rings could interact with siloxane planes on both sides at once. This was more difficult with cations such as Na+ that induced swelling and easier with those, such as K+, that were not strongly hydrated. Thus, the experimental finding was similar to that of Aggarwal et al. (2006), but received a different explanation. The authors also noted that pyrophyllite, without any interlayer cations, probably provided the best conditions to guarantee an interlayer hydrophobic environment. In a recent study, Belzunces et al. (2017) explored different interaction modes of Atrazine on Ca2+-Mt: coordination of Cl to the Ca2+ cation provided the strongest adsorption, and on a pyrophyllite surface lacking these cations atrazine molecules were only weakly adsorbed.

7.5

Adsorption: A gateway to reactivity

As already mentioned, molecules adsorbed on clay minerals may be subject to chemical reactions that do not occur, or occur only slowly, in water solution.

220

Surface and Interface Chemistry of Clay Minerals

Clay minerals are indeed good catalysts (Kloprogge, 1998; Vaccari, 1998), and they have been used in the industry for a long time, especially for acid-catalysed reactions; some clay minerals may also act as redox catalysts, especially if they contain Fe. The first step in heterogeneously catalysed mechanisms is usually the adsorption of one or several reagents on the surface, and this is why adsorption cannot be separated from reactivity. Under ambient conditions, Fe3+-Mt catalysed the in situ formation of OCDD (octachloro-p-dibenzodioxin) from pentachlorophenol (Gu et al., 2008):

The joint IR and DFT study suggested that the mechanism involved reversible, water activity-dependent coordination of the phenol to Fe3+, followed by one-electron transfer forming a phenol radical. While this transformation is detrimental because it forms a very toxic compound, similar chemistry can lead to beneficial effects. DDT is a persistent pollutant whose degradation is very difficult. It has long been known that the interaction with bentonite or vermiculite favours its activation by dehydrohalogenation (loss of HCl) (Lo´pez-Gonza´lez and Valenzuela-Calahorro, 1970):

Benzene in Cu2+-Mt is very reactive, giving rise to polymerisation products upon drying, but also phenols and other oxidation products, and dearomatisation. The NMR study by Hinedi et al. (1993) has already been mentioned; the Cu2+ ions were essential for benzene degradation, and the presence of water inhibited their activity, no doubt by preventing access to the coordination sphere (Joseph-Ezra et al., 2014). Electron transfer to Cu2+ was hypothesised to produce a radical cation susceptible to polymerisation and oxidation; phenanthrene was more reactive than pyrene. Electron transfer steps such as the ones postulated herein may initiate other catalytic reactions. The endocrine disruptor 17β-estradiol underwent oxidative polymerisation following a one-electron transfer to Fe3+ in an ion-exchanged Mt (Qin et al., 2015); the Fe2+ ions formed in the initial step were then reoxidised by atmospheric O2, providing a closed reaction sequence. The final oxidant could also be an intentionally introduced chemical such as hydrogen peroxide (H2O2). Thus, Fe-pillared clays (Fe-PILC) were active for methyl orange discolouration by catalytic wet peroxide oxidation (CWPO) (Mun˜oz et al., 2017).

Organic pollutant adsorption on clay minerals

221

This can be considered as a Fenton reaction, where Fex+ catalyses oxidation of organic matter by H2O2. Ye and Lemley (2008) compared the Fenton degradation of three pesticides (anionic, neutral, and cationic) on SWy-2 Mt. Paraquat was protected by the smectite against oxidation by the hydroxyl radicals because it was more strongly adsorbed than the other molecules. There are many reports of the use of clay minerals or modified clay minerals as Fenton catalysts (Garrido-Ramirez et al., 2010) and they cannot all be mentioned here. Other transformations more likely involve acid catalysis. In a typical example (Sa´nchez Camazano and Sa´nchez Martı´n, 1991), on Ca2+-hectorite, the organophosphate azinphosmethyl was hydrolysed along

The molecule was activated for nucleophilic attack by coordination to the interlayer cations (Lewis acid catalysis); related smectites with higher layer charges were not efficient for hydrolysis, presumably due to interlayer crowding. Wei et al. (2001) observed the hydrolysis of several carbamates such as Carbosulfan, where the addition of Mt as a catalyst increased the reaction rate more than 200-fold; illite, vermiculite, and beidellite were less efficient. The catalysis was assigned either to Br€onsted or to Lewis acidity (resp. strong H-bonding or metal ion coordination of the molecule). It is noteworthy that an analogue of carbamates with a somewhat different structure was actually protected against hydrolysis by Mt rather than activated, underlining the possible complexity of these effects. In Al-pillared Mt, the tertiary amide group of Alachlor was slowly hydrolysed (Nasser et al., 1997), giving a secondary amide (and not an amine). This degradation pathway is also confirmed by the work of Bosetto et al. (1993). Sometimes two different pollutants can react together in the presence of clay minerals. Tao and Maciel (1998) studied a mixture of benzene and CCl4 on Ca2+and Zn2+-Mt and kaolinite. These authors found that the metal cations could activate CCl4 (and probably other organochlorines) by abstraction of a Cl to give a carbocation, susceptible of nucleophilic attack on the aromatic; this process could be repeated with other chlorines, yielding products such as dibenzophenone and triphenylmethanol. Here, the crucial factor in the catalytic activity is the Lewis acidity of the metal cations: thus, dechlorination can be realised both by redox (cf. supra) and acidic catalysis.

222

Surface and Interface Chemistry of Clay Minerals

Some pollutants are slowly degraded by photochemical reactions, and clay minerals may also be active as photocatalysts. The photodegradation of trichloroethylene was observed on Ca2+-Mt by Tao et al. (1999), giving 40% mineralisation in 16 days—but forming toxic phosgene (COCl2, used as a chemical weapon) in addition to CO2! This involved a radical mechanism of Cl abstraction, with dichloroacetic acid as an intermediate. Photodegradation of bisphenol A was observed in the presence of Mt (Liu et al., 2008), as well as of paracetamol in the presence of Mt (Liu et al., 2010), and chloramphenicol and sulfamethoxazole on Mt, kaolinite and rectorite (Liu et al., 2011b). In this case the clay mineral containing systems underwent 40%–70% mineralisation in 3h, as opposed to hardly any degradation in the absence of clay minerals. Upon photolysis of carbofuran on Moroccan clay minerals, Mountacer et al. (2014) observed both a hydroxylation of the aromatic ring, and a hydrolysis of the carbamate CdO bond. On the other hand, photoprotection was observed in some systems: Ambrogi et al. (2014) recorded a slowing down of promethazine degradation by a factor of five as the drug was deposited on Mt. When it exists, the photodegradation effect could be used to enhance the stability of a drug in controlled delivery systems. Several such examples are listed in Yuan et al. (2013).

7.6

Conclusion

From this brief summary, and even more from the list of pollutant molecules provided in the next section, it can be seen that a lot of information is available on individual clay mineral–pollutant systems that have been studied for more than 50 years. This information is somewhat disparate, however. Many studies give quantitative data on adsorbed amounts as a function of environmental conditions, and they constitute a valuable database for practical applications, from environmental remediation to controlled release. But in order to understand the adsorption phenomenon at a fundamental level—and thus hopefully to be able one day to control it, and to predict it for new molecules—one has to combine macroscopic and molecular-level information. At the macroscopic level, one may propose a few guidelines for adsorption studies— the measurement of adsorbed amount of an organic molecule should be combined with estimates of the amount of cations released, and of the pH drift, to determine, respectively, if ion exchange occurs, and if the acid–base speciation is modified. At the molecular level, vibrational spectroscopy (IR) has long been available and allows us to observe changes of the functional groups brought about by their interaction with the surface through coordinative or H-bonding. XPS can also provide such information to a more limited degree; so can UV–visible absorption and fluorescence, but they are not applicable to all molecules. Solid-state NMR has a great potential, but has been little applied so far, and it is hoped that more NMR studies of pollutants adsorption will be undertaken in the future. Spectroscopic techniques are often ‘short-sighted’ in that they shed light only on localised interactions. It is getting rather clear, though, that pollutant adsorption on clay minerals is often a cooperative phenomenon, because many molecules bear several functional groups with different reactivities. The clay mineral surface on its side is

Organic pollutant adsorption on clay minerals

223

also a heterogeneous system able to induce different types of bonding. This is why it is difficult to obtain a complete picture of the adsorption, and especially of its energetics. In this respect, the rapid development of molecular modelling techniques is likely to advance our understanding of adsorption phenomena, as many possible conformations of the adsorbed molecule can be explored. For best results, though, theoretical modelling has to be combined with experimental investigations, and such integrated studies are still rare. As a final note, one should be wary of overgeneralising the results obtained in pollutant–clay mineral interaction studies. Due to the complexity of the adsorption process, each system is a particular case and should be studied in ists own respect, as witnessed by the opposing effects of clay minerals observed in degradation/ protection studies.

Appendix:

A list of pollutant structures

The following pages list the main organic pollutants whose adsorption has been studied on raw clay minerals, inorganic cation exchanged clay minerals, and some pillared clays. Substances are listed in alphabetical order; when several different substances belong to a generally recognised class or chemical family, this is indicated in bold, while the structures are compared under the heading corresponding to the class. 2,4-D: 2,4-Dichlorophenoxyacetic acid, selective herbicide, phenoxyalkanoic family (Sannino et al., 1997; Clausen et al., 2001; Tunega et al., 2007; Li et al., 2009; Bakhtiary et al., 2013; Werner et al., 2013)

Other phenoxyalkanoic herbicides include 2,4-DB, 2,4-DP, 2,4,5-T, and MCBA (Akc¸ay and Yurdakoc¸, 2000). 2,4,6-TCA: Trichloroaniline, used as a fungicide (Gianotti et al., 2008) or sometimes trichloroanisole

Acetaminophen: an acetanilide, trade name paracetamol, an analgesic pharmaceutical (Liu et al., 2010; Thiebault et al., 2016)

224

Surface and Interface Chemistry of Clay Minerals

Examples

Acetochlor: an acetanilide, herbicide, third most frequent herbicide in natural waters (Li et al., 2009; Tomic et al., 2016) € € and Ozcan, 2004) Acid blue 25: a dye (Ozcan

Acriflavine: an orange dye and antiseptic (Mishael et al., 1999)

Organic pollutant adsorption on clay minerals

225

Example: Metalaxyl

(compare with acetochlor, where a different functional group is underlined) Alachlor: a chloroacetanilide, herbicide (banned in Europe, second most used in the United States) (Bosetto et al., 1993; Nasser et al., 1997; Torrents and Jayasundera, 1997; Gerstl et al., 1998; Xu et al., 2001; Li et al., 2006; Tejada et al., 2017) Aldicarb: a carbamate insecticide, active substance in Temik (Supak et al., 1977; Wei et al., 2001) Alprazolam: a benzodiazepine, psychoactive pharmaceutical, tradename Xanax (Palace Carvalho et al., 2014) Ametryn, a triazine, herbicide (Pissinati de Rezende et al., 2013) Aminopyralid, picolinic acid class, herbicide, tradenames Milestone, Banish, etc. (Fast et al., 2010) Amitrole, a triazole, nonselective cropherbicide (Gu et al., 2015; Tan et al., 2015) AOPP, see fluazifop Astrazonred, a dye (Fil et al., 2014; Stawinski et al., 2017)

Atrazine, a triazine, herbicide, most common pollutant in drinking water in the United States, banned in Europe, linked to birth defects. 76 M pounds per year (Barriuso et al., 1994; Laird, 1996; Gonza´lez-Pradas et al., 1999; Konstantinou et al., 2000; Clausen et al., 2001; Sheng et al., 2001; Xu et al., 2001; Aggarwal et al., 2006; Li et al., 2006; Pissinati de Rezende et al., 2013; Park et al., 2014; Belzunces et al., 2017)

226

Surface and Interface Chemistry of Clay Minerals

Azinphos-methyl, an organophosphorous insecticide (Sa´nchez Camazano and Sa´nchez Martı´n, 1991)

Bentazone, a triazine (Clausen et al., 2001) Benzimidazole (Lombardi et al., 2006)

Benzodiazepins: a class of psychoactive pharmaceuticals including Valium

Examples:

Organic pollutant adsorption on clay minerals

227

Bisphenol A: used to manufacture epoxide resin. Endocrine-disrupting pollutant (Atun and Sismanoglu, 1996; Shareef et al., 2006; Liu et al., 2008; Styszko et al., 2015; Berhane et al., 2016) Carbamates, a class of insecticides inhibiting acetylcholinesterase,

Examples:

Carbamazepin, trade name Tegretol, generic pharmaceutical (psychoactive) (Drillia et al., 2005; Cabrera-Lafaurie et al., 2015; Thiebault et al., 2016)

Carbaryl, a carbamate, trade name Sevin (Sheng et al., 2001; Ye and Lemley, 2008) Carbendazim, a benzimidazole fungicide (Dios Cancela et al., 1992)

228

Surface and Interface Chemistry of Clay Minerals

Carbofuran: a carbamate, systemic insecticide, banned in Europe and Canada (Wei et al., 2001; Mountacer et al., 2014) Carbosulfan: a carbamate, systemic insecticide, banned in Europe (Wei et al., 2001) Chloramphenicol: an antibiotic (Liu et al., 2011b)

Chloroanilin: important chemical intermediate, building block of several pesticides, including chlorhexidine

4-chloro (Angioi et al., 2005) isomer; 3-chloro, 4-chloro, 3,4-dichloro (Szczepanik et al., 2014); also see TCA) Chlorpyrifos, an organophosphate pesticide (Seger and Maciel, 2006)

Ciprofloxacin: a fluoroquinolone (fluoroquinolone carboxylic acid) (Nowara et al., 1997; Wu et al., 2010; Wang et al., 2011; Li et al., 2011b; Wu et al., 2013b; Berhane et al., 2016; dos Santos et al., 2017) Clofibric acid, a herbicide, auxine antagonist (and metabolite of the cholesterollowering drug clofibrate) (Cabrera-Lafaurie et al., 2015)

Organic pollutant adsorption on clay minerals

229

Congo Red, a dye (and pH indicator), carcinogenic (Vimonses et al., 2009)

Crystal violet or Methyl violet, a dye, also general antiseptic (Mishael et al., 1999; Rytwo et al., 2009; Zhu et al., 2014; Yang et al., 2018) N

Cl−

N

N+

Cypermethrine: an insecticide of the pyrethroid class (synthetic molecules related to the naturally occurring pyrethrines); (Oudou and Hansen, 2002)

DCBN or Dichlobenil (2,6-dichlorobenzonitrile) a herbicide interfering with cellulose synthesis (Sheng et al., 2001; Li et al., 2006)

DCMU, tradename Diuron, a phenylurea, herbicide (photosynthesis inhibitor) (Sheng et al., 2001; Li et al., 2006; Maqueda et al., 2013)

230

Surface and Interface Chemistry of Clay Minerals

DDT: dichlorodiphenyltrichloroethane, an organochlorine pesticide (Lo´pezGonza´lez and Valenzuela-Calahorro, 1970; Dai et al., 2008)

Dexamethasone, a synthetic hormone, corticosteroid family (Fraile et al., 2016) Diazepam, a benzodiazepine (Palace Carvalho et al., 2014) Dibenzofurans: The basis compound is by itself harmless, but polychlorinated dibenzofurans (waste products of chlorinated compounds pyrolysis) are toxic (Chatterjee et al., 2002)

Diclofenac, pharmaceutical, anti-inflammatory, anionic pollutant, trade name Voltaren (Drillia et al., 2005; Styszko et al., 2015; Thiebault et al., 2016)

Dioxins Dioxins

, dibenzodioxins

are indus-

trial byproducts, from the combustion of plastics include TCDD, OCDD (tetra- and octachloro), and corresponding Bromo compounds (TBDD) (Chatterjee et al., 2002). Diphenhydramine: an antihistaminic drug (Li et al., 2011a)

Organic pollutant adsorption on clay minerals

231

Diquat, bipyridilium halide, nonselective herbicide, trade names Aquacid, Dectrone, etc. (Rytwo et al., 2002)

Also see paraquat

DNB, dinitrobenzene (Johnston et al., 2001) DNOC, dinitroorthocresol, a herbicide (Johnston et al., 2001; Sheng et al., 2001, 2002)

DNP, dinitrophenol (Sheng et al., 2002) The sec-butyl derivative of DNP has the trade name Dinoseb (Johnston et al., 2001)

Donepezil, a drug used in the treatment of Alzheimer disease (Park et al., 2008)

232

Surface and Interface Chemistry of Clay Minerals

Doxepin: a tricyclic, antidepressant (Thiebault et al., 2015, 2016)

Enrofloxacin, a fluoroquinolone (Nowara et al., 1997; Sturini et al., 2015) Fluazifop-p-butyl: a propionate, selective herbicide (Fusi et al., 1988) NB: more specifically an AOPP, aryloxyphenoxy propionate Flumioxazin: N-phenylphthalimide herbicide (Ferrell et al., 2005)

Gemfibrozil, pharmaceutical brand name Lopid: a fibrate (amphiphilic carboxylic acid), used to lower lipid levels

(Thiebault et al., 2016; Dordio et al.,

2017) Glyphosate: an organophosphorous herbicide, trade name Roundup (Gimsing and Borggaard, 2002; Damonte et al., 2007; Pessagno et al., 2008; Khoury et al., 2010)

Organic pollutant adsorption on clay minerals

233

Ibuprofen, a propionic acid class pharmaceutical, antiinflammatory drug, trade names Brufen, Alvin, Motril (Behera et al., 2012; Styszko et al., 2015; Thiebault et al., 2016) Imazaquin: an imidazolinone herbicide (Undabeytia et al., 2013; Park et al., 2014) Imidacloprid: a neonicotinoid insecticide. Little toxicity for mammals (Cox et al., 1998; Gonza´lez-Pradas et al., 1999; Cox et al., 2001; Liu et al., 2002)

Imidazolinone: a heterocyclic structure

Example Imazaquin

Isoproturon: a dimethylurea (Gonza´lez-Pradas et al., 1999; Clausen et al., 2001) Ketoprofen: propionic acid class antiinflammatory drug (Styszko et al., 2015; Thiebault et al., 2016) Levofloxacin, a fluroquinolone (Nowara et al., 1997; Liu et al., 2015b)

234

Surface and Interface Chemistry of Clay Minerals

Linuron, a phenylurea class herbicide, inhibitor of photosystem II (Torrents and Jayasundera, 1997) Malachite green: a cationic dye used for food colouring (and pH indicator), also biocide in aquaculture, antihelminthic, medical disinfectant. Reported to be a carcinogenic and mutagenic compound (Tahir and Rau, 2006; Kiani et al., 2011; Elmoubarki et al., 2015)

compare with methyl green and crystal violet Mecoprop, a general purpose herbicide considered as anionic (Clausen et al., 2001; Ye and Lemley, 2008)

Mefenamic acid: a pharmaceutical, painkiller (Dordio et al., 2017)

Metalaxyl: a fungicide, trade name Ridomil Gold or Mefenoxam (Wanyika, 2014)

Organic pollutant adsorption on clay minerals

235

Methyl green: a dye, used for staining DNA (Rytwo et al., 2002, 2009)

compare with malachite green and crystal violet Methyl violet, see crystal violet Methyl orange, a diazodye (Elmoubarki et al., 2015; Mun˜oz et al., 2017; Yang et al., 2018)

Methyleneblue, or MB, a dye (Bujda´k and Komadel, 1997; Mishael et al., 1999; G€urses et al., 2004; Elmoubarki et al., 2015; Stawinski et al., 2017; Fang et al., 2018; Yang et al., 2018)

Metolachlor: an acetanilide, enantioselective (S isomer is active), herbicide. Also exhibits atropoisomerism (Torrents and Jayasundera, 1997) Metsulfuron-methyl: a sulfonylurea, inhibits cell division in shoots Metribuzin: a triazine class herbicide, photosynthesis inhibitor (Aggarwal et al., 2006) Molinate: anazepam, commercial pesticide

Nalidixic acid: a quinolone antibiotic (Wu et al., 2013a)

236

Surface and Interface Chemistry of Clay Minerals

Naproxen: a propionic acid class nonsteroidal antiinflammatory drug (NSAID; Thiebault et al., 2016; Dordio et al., 2017) Nitrophenols: pollutants, from explosive and fabrics factories (Boyd et al., 2001) Example: p-nitrophenol (Park et al., 2013) Orange II: a dye (Fang et al., 2018)

PAH: polyaromatic hydrocarbons Examples:

Paraquat: a viologen, cationic pesticide, nonselective herbicide, involved in Parkinson’s disease (Rytwo et al., 2002; Ye and Lemley, 2008; Gu et al., 2015)

Parathion, an organophosphate, insecticide, acaricide (Sheng et al., 2001)

Phenanthrene, a PAH pollutant (Joseph-Ezra et al., 2014) Phosmet, a phthalimide organophosphate (Sa´nchez Camazano and Sa´nchez Martı´n, 1980)

Organic pollutant adsorption on clay minerals

237

Picloram: a herbicide of the picolinic acid class, brand name Tordon, present in Agent White and Agent Orange (Fast et al., 2010; Marco-Brown et al., 2012, 2014, 2015, 2017) Picolinic acids, a class of herbicides, based on the core structure

Examples:

Piroxicam: a NSAID (nonsteroid antiinflammatory drug), sensitive to light (Ambrogi et al., 2012)

Promethazine (Ambrogi et al., 2014)

238

Surface and Interface Chemistry of Clay Minerals

Prometryn, a triazine, herbicide (Konstantinou et al., 2000; Grabka et al., 2017) Propachlor, an acetanilide, herbicide, forbidden in Europe (Konstantinou et al., 2000) Propanil, an acetanilide, herbicide, several thousand tonnes per year (Konstantinou et al., 2000) Propionates, or propionic acid class, a family of drugs containing the group

Examples

Pyrene: a PAH pollutant (Joseph-Ezra et al., 2014) Pyronin B, a dye (Grauer et al., 1987a)

Quinolones: a class of antibiotics, core structure

Organic pollutant adsorption on clay minerals

239

Examples:

ciprofloxacine, enrofloxacine, levofloxacine, and norfloxacineare fluoroquinolones Rhodamine B, a dye, zwitterionic (Grauer et al., 1987b; Fang et al., 2018)

Salicylic acid, a pharmaceutical, trade name Aspirin (Cabrera-Lafaurie et al., 2015)

240

Surface and Interface Chemistry of Clay Minerals

Simazine: a triazine class herbicide, trade names Gesatop, Princep, (Aggarwal et al., 2006; Pissinati de Rezende et al., 2013) Steroid hormones: endocrine disruptors, for example, from birth control pills (Shareef et al., 2006) Examples: Dexamethasone

Estrone

17α ethynylestradione

Sulfamethazine, a sulfonamide antibiotic (Gao and Pedersen, 2005) Sulfamethoxazole, a sulfonamide antibiotic (Liu et al., 2011b) Sulfathiazole, a sulfonamide, antimicrobial (aquariums) (Kahle and Stamm, 2007) Sulfonamides or sulfa drugs: a class of antibacterial substances that also includes other types of drugs, based on the –SO2–NR2 moiety (see also ureas, sulfonylurea) Examples: Sulfamethazine

Sulfamethoxazole

Sulfathiazole

(the imino form is shown; it is in equilibrium with an amino form)

Organic pollutant adsorption on clay minerals

241

Sulforhodamine G: a dye, anionic (Fang et al., 2018)

Tetracyclins: octahydrotetracene-2-carboxamide derivatives (Figueroa et al., 2004): includes basic compound + oxytetracycline and chlortetracycline Examples: Chlortetracycline

Tetracycline (Liu et al., 2011b)

Thiabendazole: a fungicide (Lombardi et al., 2006)

Oxytetracycline

242

Surface and Interface Chemistry of Clay Minerals

Tramadol: opioid of the ‘benzenoid class’, pharmaceutical, brand name Ultram (Thiebault et al., 2015, 2016)

NB: the (1R, 2R) isomer is shown

Triazines: a class of pesticides based on a heterocyclic nucleus; comparison of 6¼ triazines (Weber, 1966; Zarpon et al., 2006)

, sometimes other isomers

Examples: Ametryn

Metribuzine

Bentazone

Prometryne

Atrazine

Simazine

Organic pollutant adsorption on clay minerals

243

Triazoles: a class of pesticides based on a five-membered heterocyclic nucleus

Example: Amitrole

Trichloroethylene, an industrial solvent (Tao et al., 1999) Triclopyr: a pyridine, insecticide, and fungicide (Pusino et al., 1994)

Triclosan: antibacterial, antifungal, also found in PCP and toys (Styszko et al., 2015)

Urea: core structure

, present in several herbicides

244

Surface and Interface Chemistry of Clay Minerals

Examples: DCMU, a phenylurea

Linuron, a phenylurea

Isoproturon, a phenylurea (both also dimethylureas)

Metsulfuron-methyl, a sulfonylurea (also a triazine)

References Aggarwal, V., Li, H., Teppen, B.J., 2006. Triazine adsorption by saponite and beidellite clay minerals. Environ. Sci. Technol. 25 (2), 392–399. Akc¸ay, G., Yurdakoc¸, K., 2000. Removal of various phenoxyalkanoic acid herbicides from water by organo-clays. Acta Hydrochim. Hydrobiol. 28 (6), 300–304. Ambrogi, V., Latterini, L., Nocchetti, M., Pagano, C., Ricci, M., 2012. Montmorillonite as an agent for drug photostability. J. Mater. Chem. 22, 22743. Ambrogi, V., Nocchetti, M., Latterini, L., 2014. Promethazine montmorillonite inclusion complex to enhance drug photostability. Langmuir 30, 14612–14620. Angioi, S., Polati, S., Roz, M., Rinaudo, C., Gianotti, V., Gennaro, M.C., 2005. Sorption studies of chloroanilines on kaolinite and montmorillonite. Environ. Pollut. 134, 35–43. Arias-Est
evez, M., Lo´pez-Periago, E., Martı´nez-Carballo, E., Simal-Gandara, J., Garcı´aRı´o, J.-C.M.L., 2008. The mobility and degradation of pesticides in soils and the pollution of groundwater resources. Agric. Ecosyst. Environ. 123, 247–260. Atun, G., Sismanoglu, T., 1996. Adsorption of 4,40 -iso propylidene diphenol and diphenylolpropane 4,40 dioxyacetic acid from aqueous solution on kaolinite. J. Environ. Sci. Health A 31 (8), 2055–2069. Bakhtiary, S., Shirvani, M., Shariatmadari, H., 2013. Adsorption–desorption behavior of 2,4-D on NCP-modified bentonite and zeolite: implications for slow-release herbicide formulations. Chemosphere 90, 699–705. Barriuso, E., Laird, D.A., Koskinen, W.C., Dowdy, R.H., 1994. Atrazine desorption from smectites. Soil Sci. Soc. Am. J. 58, 1632–1638. Behera, S.K., Oh, S.Y., Park, H.S., 2012. Sorptive removal of ibuprofen from water using selected soil minerals and activated carbon. Int. J. Environ. Sci. Technol. 9, 85–94.

Organic pollutant adsorption on clay minerals

245

Belzunces, B., Hoyau, S., Benoit, M., Tarrat, N., Bessac, F., 2017. Theoretical study of the atrazine pesticide interaction with pyrophyllite and Ca2+ montmorillonite clay surfaces. J. Comput. Chem. 38, 133–143. Berhane, T.M., Levy, J., Krekeler, M.P.S., Danielson, N.D., 2016. Adsorption of bisphenol A and ciprofloxacin by palygorskite-montmorillonite: effect of granule size, solution chemistry and temperature. Appl. Clay Sci. 132–133, 518–527. Biache, C., Lorgeoux, C., Saad, A., Faure, P., 2015. Behavior of PAH/mineral associations during thermodesorption: impact for the determination of mineral retention properties towards PAHs. Anal. Bioanal. Chem. 407, 3509–3516. Bonhomme, C., Gervais, C., Laurencin, D., 2014. Recent NMR developments applied to organic–inorganic materials. Progr. Nucl. Magn. Res. Spectr. 77, 1–48. Bosetto, M., Arfaioli, P., Fusi, P., 1993. Interactions of alachlor with homoionic montmorillonites. Soil Sci. 155 (2), 105–113. Bosetto, M., Arfaioli, P., Fusi, P., 1994. Adsorption of metolachlor on homoionic montmorillonites. Agrochimica 38 (1–2), 14–24. Boyd, S.A., Sheng, G.Y., Teppen, B.J., Johnston, C.T., 2001. Mechanisms for the adsorption of substituted nitrobenzenes by smectite clays. Environ. Sci. Technol. 35, 4227–4234. Boyd, S.A., Johnston, C.T., Laird, D.A., Teppen, B.J., Li, H., 2011a. Comprehensive study of organic contaminant adsorption by clays: methodologies, mechanisms, and environmental implications. In: Biophysico-Chemical Processes of Anthropogenic Organic Compounds in Environmental Systems. John Wiley & Sons, Inc. Boyd, S.A., Johnston, C.T., Pinnavaia, T.J., Kaminski, N.E., Teppen, B.J., Li, H., Khan, B., Crawford, R.B., Kovalova, N., Kim, S.-S., Shao, H., Gu, C., Kaplan, B.L.F., 2011b. Suppression of humoral immune responses by 2,3,7,8-tetrachlorodibenzo-p-dioxin intercalated in smectite clay. Environ. Toxicol. Chem. 30 (12), 2748–2755. Bruzzi, E., Stace, A.J., 2017. Experimental measurements of water molecule binding energies for the second and third solvation shells of [Ca(H2O)n]2+ complexes. R. Soc. Open Sci. 4(1), 160671. Bujda´k, J., Komadel, P., 1997. Interaction of methylene blue with reduced charge montmorillonite. J. Phys. Chem. B 101, 9065–9068. Cabrera-Lafaurie, W.A., Roma´n, F.R., Herna´ndez-Maldonado, A.J., 2015. Single and multicomponent adsorption of salicylic acid, clofibric acid, carbamazepine and caffeine from water onto transition metal modified and partially calcined inorganic–organic pillared clay fixed beds. J. Hazard. Mater. 282, 174–182. Chatterjee, A., Iwasaki, T., Ebina, T., 2002. 2:1 dioctahedral smectites as a selective sorbent for ioxins and furans: reactivity index study. J. Phys. Chem. A 106, 641–648. Chavez, P., Ducker, W., Israelachvili, J., Maxwell, K., 1996. Adsorption of dipolar (zwitterionic) surfactants to dipolar surfaces. Langmuir 12, 4111–4115. Clausen, L., Fabricius, I., Madsen, L., 2001. Adsorption of pesticides onto quartz, calcite, kaolinite, and α-alumina. J. Environ. Qual. 30, 846–857. Cohen, R., Yariv, S., 1984. Metachromasy in clay minerals—sorption of acridine orange by montmorillonite. J. Chem. Soc. Faraday Trans. 1 (80), 1705–1715. Collins, J.R., Loew, G.H., Luke, B.T., White, D.H., 1988. Theoretical investigation of the role of clay edges in prebiotic peptide bond formation. Orig. Life Evol. Biosph. 18, 107–119. Cornejo, J., Celis, R., Pavlovic, I., Ulibarri, M.A., 2008. Interactions of pesticides with clays and layered double hydroxides: a review. Clay Miner. 43, 155–175. Cox, L., Koskinen, W.C., Celis, R., Yen, P.Y., Hermosin, M.C., Cornejo, J., 1998. Sorption of imidacloprid on soil clay mineral and organic components. Soil Sci. Soc. Am. J. 62 (4), 911–915.

246

Surface and Interface Chemistry of Clay Minerals

Cox, L., Hermosin, M.C., Koskinen, W.C., Cornejo, J., 2001. Interactions of imidacloprid with organic and inorganic-exchanged smectites. Clay Miner. 36, 267–274. Dai, R.-L., Zhang, G.-Y., Gu, X.-Z., Wang, M.K., 2008. Sorption of 1,1,1-trichloro-2,2-bis (p-chlorophenyl) ethane (DDT) by clays and organoclays. Environ. Geochem. Health 30, 479–488. Damonte, M., Sa´nchez, R.M.T., Afonso, M.d.S., 2007. Some aspects of the glyphosate adsorption on montmorillonite and its calcined form. Appl. Clay Sci. 36, 86–94. Delle Site, A., 2001. Factors affecting sorption of organic compounds in natural sorbent/water systems and sorption coefficients for selected pollutants. A review. J. Phys. Chem. Ref. Data 30 (1), 187–439. Dios Cancela, G., Romero Taboada, E., Sa´nchez-Rasero, F., 1992. Carbendazim adsorption on montmorillonite, peat and soils. J. Soil Sci. 99–111. Dordio, A.V., Miranda, S., Prates Ramalhoa, J.P., Palace Carvalhoa, A.J., 2017. Mechanisms of removal of three widespread pharmaceuticals by two clay materials. J. Hazard. Mater. 323, 575–583. dos Santos, E.C., Rozynek, Z., Hansen, E.L., Hartmann-Petersen, R., Klitgaard, R.N., LøbnerOlesen, A., Michels, L., Mikkelsen, A., Plivelic, T.S., Bordallo, H.N., Fossum, J.O., 2017. Ciprofloxacin intercalated in fluorohectorite clay: identical pure drug activity and toxicity with higher adsorption and controlled release rate. RSC Adv. 7, 26537–26545. Dougherty, D.A., 1996. Cation–π interactions in chemistry and biology: a new view of benzene, Phe, Tyr, and Trp. Science 271 (5246), 163–168. Drillia, P., Stamatelatou, K., Lyberatos, G., 2005. Fate and mobility of pharmaceuticals in solid matrices. Chemosphere 60, 1034–1044. Elmoubarki, R., Mahjoubi, F.Z., Tounsadi, H., Moustadraf, J., Abdennouri, M., Zouhri, A., El Albani, A., Barka, N., 2015. Adsorption of textile dyes on raw and decanted Moroccan clays: kinetics, equilibrium and thermodynamics. Water Res. Ind. 9, 16–29. Fang, Y.F., Zhou, A., Yang, W., Araya, T., Huang, Y.P., Zhao, P., Johnson, D., Wang, J.Z., Ren, Z.Y.J., 2018. Complex formation via hydrogen bonding between rhodamine B and montmorillonite in aqueous solution. Sci. Rep. 8, 229. Fast, B.J., Ferrell, J.A., MacDonald, G.E., Krutz, L.J., Kline, W.N., 2010. Picloram and aminopyralid sorption to soil and clay minerals. Weed Sci. 58 (4), 484–489. Ferrell, J. A., Vencill, W. K., Xia, K., Grey, T. L., 2005. Sorption and desorption of flumioxazin to soil, clay minerals and ion-exchange resin. 61, 40–46. Figueroa, R.A., Leonard, A., MacKay, A.A., 2004. Modeling tetracycline antibiotic sorption to clays. Environ. Sci. Technol. 38, 476–483. Fil, B.A., Yilmaz, M.T., Bayar, S., Elkoca, M.T., 2014. Investigation of adsorption of the dyestuff astrazon red violet 3RN (basic violet 16) on montmorillonite clay. Braz. J. Chem. Eng. 31 (1), 171–182. Fraile, J.M., Garcia-Martin, E., Gil, C., Mayoral, J.A., Pablo, L.E., Polo, V., Prieto, E., Vispe, E., 2016. Laponite as carrier for controlled in vitro delivery of dexamethasone in vitreous humor models. Eur. J. Pharm. Biopharm. 108, 83–90. Fusi, P., Franci, M., Bosetto, M., 1988. Interaction of fluazifop-butyl and fluazifop with smectites. Appl. Clay Sci. 3, 63–73. Gao, J.A., Pedersen, J.A., 2005. Adsorption of sulfonamide antimicrobial agents to clay minerals. Environ. Sci. Technol. 39, 9509–9516. Garrido-Ramirez, E.G., Theng, B.K.G., Mora, M.L., 2010. Clays and oxide minerals as catalysts and nanocatalysts in fenton-like reactions—a review. Appl. Clay Sci. 47 (3–4), 182–192. Gerstl, Z., Nasser, A., Mingelgrin, U., 1998. Controlled release of pesticides into soils from clay–polymer formulations. J. Agric. Food Chem. 46, 3797–3802.

Organic pollutant adsorption on clay minerals

247

Gevao, B., Semple, K.T., Jones, K.C., 2000. Bound pesticide residues in soils: a review. Environ. Pollut. 108, 3–14. Gianotti, V., Benzi, M., Croce, G., Frascarolo, P., Gosetti, F., Mazzucco, E., Bottaro, M., Gennaro, M.C., 2008. The use of clays to sequestrate organic pollutants. Leaching experiments. Chemosphere 73, 1731–1736. Gimsing, A.L., Borggaard, O.K., 2002. Competitive adsorption and desorption of glyphosate and phosphate on clay silicates and oxides. Clay Min. 37, 509–515. Gonza´lez-Pradas, E., Villafranca-Sa´nchez, M., Socı´as-Viciana, M., Ferna´ndez-P
erez, M., Urena-Amate, M., 1999. Preliminary studies in removing atrazine, isoproturon and imidacloprid from water by natural sepiolite. J. Chem. Technol. Biotechnol. 74, 417–422. Grabka, D., Raczy
nska-Z˙ak, M., Czech, K., Słomkiewicz, P.M., Jo´z´wiak, M.A., 2017. Modified halloysite as an adsorbent for prometryn from aqueous solutions. Appl. Clay Sci. 114, 321–329. Grauer, Z., Grauer, G.L., Avnir, D., Yariv, S., 1987a. Metachromasy in clay minerals—sorption of pyronin-Y by montmorillonite and laponite. J. Chem. Soc. Faraday Trans. 1 (83), 1685–1701. Grauer, Z., Malter, A.B., Yariv, S., Avnir, D., 1987b. Sorption of rhodamine B by montmorillonite and laponite. Colloids Surf. 25, 41–46. Gu, C., Li, H., Teppen, B.J., Boyd, S.A., 2008. Octachlorodibenzodioxin formation on Fe(III)montmorillonite clay. Environ. Sci. Technol. 42, 4758–4763. Gu, Z., Gao, M.L., Lu, L.F., Liu, Y.N., Yang, S.F., 2015. Montmorillonite functionalized with zwitterionic surfactant as a highly efficient adsorbent for herbicides. Ind. Eng. Chem. Res. 54, 4947–4955. G€ urses, A., Karaca, S., D€ogar, C ¸ ., Bayrak, R., Ac¸ıkyıldız, M., Yalc¸ın, M., 2004. Determination of adsorptive properties of clay/water system: methylene blue sorption. J. Colloid Interface Sci. 269, 310–314. Hang, P.T., Brindley, G.W., 1970. Methylene blue adsorption by clay minerals. II (clay-organic studies XVIII). Clays Clay Miner. 18, 203–212. Hinedi, Z.R., Johnston, C.T., Erickson, C., 1993. Chemisorption of benzene on Cu-montmorillonite as characterized by FTIR and 13C MAS NMR. Clays Clay Miner. 41, 87–94. Ho, Y.S., McKay, G., 1999. Pseudo-second order model for sorption processes. Process Biochem. 34, 451–465. Hou, X.-J., Li, H.Q., He, P., Li, S.P., Liu, Q.F., 2015. Molecular-level investigation of the adsorption mechanisms of toluene and aniline on natural and organically modified montmorillonite. J. Phys. Chem. A 119, 11199–11207. Ibarguren, C., Naranjo, P.M., St€otzel, C., Audisio, M.C., Shama, E.L., Farfa´n Torres, E.M., M€uller, F.A., 2014. Adsorption of nisin on raw montmorillonite. Appl. Clay Sci. 90, 88–95. Jacobs, P.A., Theng, B.K.G., Uytterhoeven, J.B., 1972. Quantitative infrared spectroscopy of amines in synthetic zeolites X and Y. II. Adsorption of amines on Na-hydrogen zeolites X and Y. J. Catal. 26, 191–201. Johnston, C.T., 1996. Sorption of organic compound on clay minerals: a surface functional group approach. In: Sawhney, B. (Ed.), Organic Pollutants in Soil Environments. Clay Minerals Society, Boulder, CO. Johnston, C.T., De Oliveira, M.F., Teppen, B.J., Sheng, G.Y., Boyd, S.A., 2001. Spectroscopic study of nitroaromatic-smectite sorption mechanisms. Environ. Sci. Techol. 35, 4767–4772.

248

Surface and Interface Chemistry of Clay Minerals

Joseph-Ezra, H., Nasser, A., Mingelgrin, U., 2014. Surface interactions of pyrene and phenanthrene on Cu-montmorillonite. Appl. Clay Sci. 95, 348–356. Kahle, M., Stamm, C., 2007. Time and pH-dependent sorption of the veterinary antimicrobial sulfathiazole to clay minerals and ferrihydrite. Chemosphere 68, 1224–1231. Kaya, E.M.O., Ozcan, A.S., G€ok, O., Ozcan, A., 2013. Adsorption kinetics and isotherm parameters of naphthalene onto natural- and chemically modified bentonite from aqueous solutions. Adsorption 19, 879–888. Khoury, G.A., Gehris, T.C., Tribe, L., Sa´nchez, R.M.T., Afonso, M.d.S., 2010. Glyphosate adsorption on montmorillonite: an experimental and theoretical study of surface complexes. Appl. Clay Sci. 50, 167–175. Kiani, G., Dostali, M., Rostami, A., Khataee, A.R., 2011. Adsorption studies on the removal of Malachite Green from aqueous solutions onto halloysite nanotubes. Appl. Clay Sci. 54, 34–39. Kloprogge, J.T., 1998. Synthesis of smectites and porous pillared clay catalysts: a review. J. Porous Mater. 5 (1), 5–41. Konstantinou, I.K., Albanis, T.A., Dimitrios, M., Petrakis, E., Pomonis, P.J., 2000. Removal of herbicides from aqueous solutions by adsorption on Al-pillared clays, Fe–Al pillared clays and mesoporous alumina aluminum phosphates. Water Res. 34 (12), 3123–3136. Koutsopoulou, E., Papoulis, D., Tsolis-Katagas, P., Kornaros, M., 2010. Clay minerals used in sanitary landfills for the retention of organic and inorganic pollutants. Appl. Clay Sci. 49, 372–382. Kulik, D.A., 2009. Thermodynamic concepts in modeling sorption at the mineral–water interface. Rev. Mineral. Geochem. 70 (1), 125–180. Lagaly, G., 2001. Pesticide–clay interactions and formulations. Appl. Clay Sci. 18, 205–209. Laird, D.A., 1996. Interactions between atrazine and smectite surfaces. In: Meyer, M.T., Thurman, E.M. (Eds.), Herbicide Metabolites in Surface Water and Groundwater. American Chemical Society, Washington, DC. Lan, T., Kaviratna, D., Pinnavaia, T. J., 1996. Epoxy self-polymerization in smectite clays. 57, 6–8, 1005-1010. Li, H., Teppen, B.J., Laird, D.A., Johnston, C.T., Boyd, S.A., 2006. Effects of increasing potassium chloride and calcium chloride ionic strength on pesticide sorption by potassium- and calcium-smectite. Soil Sci. Soc. Am. J. 70 (6), 1889–1895. Li, J.F., Li, Y.M., Lu, J.H., 2009. Adsorption of herbicides 2,4-D and acetochlor on inorganic– organic bentonites. Appl. Clay Sci. 46, 314–318. Li, Z.H., Chang, P.-H., Jiang, W.-T., Jean, J.-S., Hong, H.L., Liao, L.B., 2011a. Removal of diphenhydramine from water by swelling clay minerals. J. Colloid Interface Sci. 360, 227–232. Li, Z.H., Hong, H.L., Liao, L.B., Ackley, C.J., Schulz, L.A., MacDonald, R.A., Mihelich, A.L., Emard, S.M., 2011b. A mechanistic study of ciprofloxacin removal by kaolinite. Colloids Surf. B 88, 339–344. Liu, W.P., Zheng, W., Gan, J.Y., 2002. Competitive sorption between imidacloprid and imidacloprid-urea on soil clay minerals and humic acids. J. Agric. Food Chem. 50, 6823–6827. Liu, Y.X., Zhang, X., Guo, L., Wu, F., Deng, N.S., 2008. Photodegradation of bisphenol A in the montmorillonite KSF suspended solutions. Ind. Eng. Chem. Res. 47, 7141–7146. Liu, Y.X., Wan, K., Deng, N.S., Wu, F., 2010. Photodegradation of paracetamol in montmorillonite KSF suspension. React. Kinet. Catal. Lett. 99, 493–502. Liu, R.C., Zhang, B., Mei, D.D., Zhang, H.Q., Liu, J.D., 2011a. Adsorption of methyl violet from aqueous solution by halloysite nanotubes. Desalination 268, 111–116.

Organic pollutant adsorption on clay minerals

249

Liu, Y.X., Lu, X.J., Wu, F., Deng, N.S., 2011b. Adsorption and photooxidation of pharmaceuticals and personal care products on clay minerals. React. Kinet. Catal. Lett. 104, 61–73. Liu, C., Johnston, C.T., Boyd, S.A., Teppen, B.J., 2012. Relating clay structural factors to dioxin adsorption by smectites: molecular dynamics simulations. Soil Sci. Soc. Am. J. 76 (1), 110–120. Liu, C., Gu, C., Yu, K., Li, H., Teppen, B.J., Johnston, C.T., Boyd, S.A., Zhou, D.M., 2015a. Integrating structural and thermodynamic mechanisms for sorption of PCBs by montmorillonite. Environ. Sci. Technol. 49, 2796–2805. Liu, Y.N., Dong, C.X., Yuan, H.W.W.H., Li, K.B., 2015b. Adsorption of levofloxacin onto an iron-pillared montmorillonite (clay mineral): kinetics, equilibrium and mechanism. Appl. Clay Sci. 118, 301–307. Lombardi, B.M., Sanchez, R.M.T., Eloy, P., Genet, M., 2006. Interaction of thiabendazole and benzimidazole with montmorillonite. Appl. Clay Sci. 33, 59–65. Lo´pez Arbeloa, F., Martı´nez Martı´nez, V., 2006. Orientation of adsorbed dyes in the interlayer space of clays. 2 fluorescence polarization of rhodamine 6G in laponite films. Chem. Mater. 18, 1407–1416. Lo´pez-Gonza´lez, J.d.D., Valenzuela-Calahorro, C., 1970. Associated decomposition of DDT to DDE in the diffusion of DDT on homoionic clays. J. Agric. Food Chem. 19 (3), 520–523. Maqueda, C., dos Santos Afonso, M., Morillo, E., Torres Sa´nchez, R.M., Perez-Sayago, M., Undabeytia, T., 2013. Adsorption of diuron on mechanically and thermally treated montmorillonite and sepiolite. Appl. Clay Sci. 72, 175–183. Marco-Brown, J.L., Barbosa-Lema, C.M., Torres Sa´nchez, R.M., Mercader, R.C., Afonso, M.d. S., 2012. Adsorption of picloram herbicide on iron oxide pillared montmorillonite. Appl. Clay Sci. 58, 25–33. Marco-Brown, J.L., Areco, M.M., Torres Sa´nchez, R.M., Afonso, M.d.S., 2014. Adsorption of picloram herbicide on montmorillonite: kinetic and equilibrium studies. Colloids Surf. A 449, 121–128. Marco-Brown, J.L., Trinelli, M.A., Gaigneaux, E.M., Torres Sa´nchez, R.M., Afonso, M.d.S., 2015. New insights on the structure of the picloram–montmorillonite surface complexes. J. Colloid Interface Sci. 444, 115–122. Marco-Brown, J.L., Undabeytia, T., Torres Sa´nchez, R.M., Afonso, M.d.S., 2017. Slow-release formulations of the herbicide picloram by using Fe–Al pillared montmorillonite. Envir. Sci. Pollut. Res. 24, 10410–10420. Meleshyn, A., Tunega, D., 2011. Adsorption of phenanthrene on Na-montmorillonite: a model study. Geoderma 169, 41–46. Mishael, Y.G., Rytwo, G., Nir, S., Crespin, M., Annabi-Bergaya, F., Damme, H.V., 1999. Interactions of monovalent organic cations with pillared clays. J. Colloid Interface Sci. 209, 123–128. Mortland, M.M., 1970. Clay–organic complexes and interactions. Adv. Agron. 22, 75–117. Mortland, M.M., Raman, K.V., 1968. Surface acidity of smectites in relation to hydration, exchangeable-cation and structure. Clays Clay Miner. 16, 393–398. Mountacer, H., Atifi, A., Wong-Wah-Chung, P., Sarakha, M., 2014. Degradation of the pesticide carbofuran on clay and soil surfaces upon sunlight exposure. Environ. Sci. Pollut. Res. 21, 3443–3451. ´ ., Galeano, L.-A., 2017. Preparation of Mun˜oz, H.-J., Blanco, C., Gil, A., Vicente, M.-A Al/Fe-pillared clays: effect of the starting mineral. Materials 10, 1364. Nasser, A., Gal, M., Gerstl, Z., Mingelgrin, U., Yariv, S., 1997. Adsorption of alachlor by montmorillonites. J. Therm. Anal. Calorim. 50, 257–268.

250

Surface and Interface Chemistry of Clay Minerals

Ngulube, T., Gumbo, J.R., Masindi, V., Maity, A., 2017. An update on synthetic dyes adsorption onto clay based minerals: a state-of-art review. J. Environ. Manag. 191, 35–57. Nir, S., El-Nahhal, Y., Undabeytia, T., Rytwo, G., Polubesova, T., Mishael, Y., Rabinovitz, O., Rubin, B., 2013. Clays, clay minerals, and pesticides. In: Lagaly, F.B.a.G. (Ed.), Handbook of Clay Science Part B: Techniques and Applications. Elsevier, Amsterdam. Nowara, A., Burhenne, J., Spiteller, M., 1997. Binding of fluoroquinolone carboxylic acid derivatives to clay minerals. J. Agric. Food Chem. 45, 1459–1463. Oudou, H.C., Hansen, H.C.B., 2002. Sorption of lambda-cyhalothrin, cypermethrin, deltamethrin and fenvalerate to quartz, corundum, kaolinite and montmorillonite. Chemosphere 49, 1285–1294. € € Ozcan, A.S., Ozcan, A., 2004. Adsorption of acid dyes from aqueous solutions onto acidactivated bentonite. J. Colloid Interface Sci. 276, 39–46. Palace Carvalho, A.J., Dordio, A.V., Prates Ramalho, J.P., 2014. A DFT study on the adsorption of benzodiazepines to vermiculite surfaces. J. Mol. Model. 20, 2336. Park, J.K., Choy, Y.B., Oh, J.-M., Kim, J.Y., Hwang, S.-J., Choy, J.-H., 2008. Controlled release of donepezil intercalated in smectite clays. Int. J. Pharm. 359, 198–204. Park, Y., Ayoko, G.A., Frost, R.L., 2011. Application of organoclays for the adsorption of recalcitrant organic molecules from aqueous media. J. Colloid Interface Sci. 354, 292–305. Park, Y., Frost, R.L., Ayoko, G.A., Morgan, D.L., 2013. Adsorption of p-nitrophenol on organoclays. A thermoanalytical study. J. Therm. Anal. Calorim. 111, 41–47. Park, Y., Sun, Z.M., Ayoko, G.A., Frost, R.L., 2014. Removal of herbicides from aqueous solutions by modified forms of montmorillonite. J. Colloid Interface Sci. 415, 127–132. Pessagno, R.C., Sa´nchez, R.M.T., Afonso, M.d.S., 2008. Glyphosate behavior at soil and mineral–water interfaces. Environ. Pollut. 153, 53–59. Pissinati de Rezende, E.I., Peralta-Zamora, P.G., Jardim, W.d.F., Vidal, C., Abate, G., 2013. Sorption and preconcentration of the herbicides atrazine, simazine, and ametryne on montmorillonite. Anal. Chem. 46 (3), 439–451. Poinsignon, C., Cases, J.M., Fripiat, J.J., 1978. Electrical-polarization of water molecules adsorbed by smectites. An infrared study. J. Phys. Chem. 82, 1855–1860. Polati, S., Angioi, S., Gianotti, V., Gosetti, F., Gennaro, M.C., 2006. Sorption of pesticides on kaolinite and montmorillonite as a function of hydrophilicity. J. Environ. Sci. Health B 41 (4), 333–344. Pusino, A., Liu, W.P., Gessa, C., 1994. Adsorption of triclopyr on soil and some of its components. J. Agric. Food Chem. 42 (4), 1026–1029. Qin, C., Troya, D., Shang, C., Hildreth, S., Helm, R., Xia, K., 2015. Surface catalyzed oxidative oligomerization of 17β-estradiol by Fe3+-saturated montmorillonite. Environ. Sci. Technol. 49, 956–964. Rana, K., Boy, S.A., Teppen, B.J., Li, H., Liu, C., Johnston, C.T., 2009. Probing the microscopic hydrophobicity of smectite surfaces. A vibrational spectroscopic study of dibenzo-pdioxin sorption to smectites. Phys. Chem. Chem. Phys. 11, 2976–2985. Rimola, A., Costa, D., Sodupe, M., Lambert, J.-F., Ugliengo, P., 2013. Silica surface features and their role in the adsorption of bio-molecules: computational modeling and experiments. Chem. Rev. 113, 4216–4313. Rytwo, G., Tropp, D., Serban, C., 2002. Adsorption of diquat, paraquat and methyl green on sepiolite: experimental results and model calculations. Appl. Clay Sci. 20, 273–282. Rytwo, G., Gonen, Y., Huterer-Shveky, R., 2009. Evidence of degradation of triarylmethine dyes on texas vermiculite. Clays Clay Miner. 57 (5), 555–565. Sabah, E., Ouki, S., 2017. Adsorption of pyrene from aqueous solutions onto sepiolite. Clays Clay Miner. 65 (1), 14–26.

Organic pollutant adsorption on clay minerals

251

Sa´nchez Camazano, M., Sa´nchez Martı´n, M.J., 1980. Interaction of phosmet with montmorillonite. Soil Sci. 129, 115–118. Sa´nchez Camazano, M., Sa´nchez Martı´n, M.J., 1991. Hydrolysis of azinphosmethyl induced by the surface of smectites. Clays Clay Miner. (6), 609–613. Sannino, F., Violante, A., Gianfreda, L., 1997. Adsorption–desorption of 2,4-D by hydroxy aluminium montmorillonite complexes. Pestic. Sci. 51, 429–435. Sarma, G.K., Gupta, S.S., Bhattacharyya, K.G., 2016. Adsorption of crystal violet on raw and acid-treated montmorillonite, K10, in aqueous suspension. J. Environ. Manag. 171, 1–10. Seger, M.R., Maciel, G.E., 2006. NMR investigation of the behavior of an organothiophosphate pesticide, chlorpyrifos, sorbed on montmorillonite clays. Environ. Sci. Technol. 40, 797–802. Shapley, T.V., Molinari, M., Zhu, R.L., Parker, S.C., 2013. Atomistic modeling of the sorption free energy of dioxins at clay–water interfaces. J. Phys. Chem. C 117, 24975–24984. Shareef, A., Angove, M.J., Wells, J.D., Johnson, B.B., 2006. Sorption of bisphenol A, 17α-ethynylestradiol and estrone to mineral surfaces. J. Colloid Interface Sci. 297, 62–69. Sheng, G., Johnston, C.T., Teppen, B.J., Boyd, S.A., 2001. Potential contributions of smectite clays and organic matter to pesticide retention in soils. J. Agric. Food Chem. 49, 2899–2907. Sheng, G.Y., Johnston, C.T., Teppen, B.J., Boyd, S.A., 2002. Adsorption of dinitrophenol herbicides from water by montmorillonites. Clays Clay Miner. 50 (1), 25–34. Stawinski, W., Wegrzyn, A., Danko, T., Freitas, O., Figueiredo, S., Chmielarz, L., 2017. Acid– base treated vermiculite as high performance adsorbent: insights into the mechanism of cationic dyes adsorption, regeneration, recyclability and stability studies. Chemosphere 173, 107–115. Sturini, M., Speltini, A., Maraschi, F., Rivagli, E., Pretali, L., Malavasi, L., Profumo, A., Fasani, E., Albini, A., 2015. Sunlight photodegradation of marbofloxacin and enrofloxacin adsorbed on clay minerals. J. Photochem. Photobiol. A 299, 103–109. Styszko, K., Nosek, K., Motak, M., Bester, K., 2015. Preliminary selection of clay minerals for the removal of pharmaceuticals, bisphenol A and triclosan in acidic and neutral aqueous solutions. C.R. Chim. 18, 1134–1142. Supak, J.R., Swoboda, A.R., Dixon, J.B., 1977. Adsorption of aldicarb by clays and soil organo– clay complexes. Soil Sci. Soc. Am. J 42 (2), 244–248. Szczepanik, B., Słomkiewicz, P., Garnuszek, M., Czech, K., 2014. Adsorption of chloroanilines from aqueous solutions on the modified halloysite. Appl. Clay Sci. 101, 260–264. Tahir, S.S., Rau, N., 2006. Removal of a cationic dye from aqueous solutions by adsorption onto bentonite clay. Chemosphere 63, 1842–1848. Tahoun, S.A., Mortland, M.M., 1966. Complexes of montmorillonite with primary, secondary and tertiary amides. 2. Coordination of amides on surface of montmorillonite. Soil Sci. 102 (5), 314. Tan, D.Y., Yuan, P., Annabi-Bergaya, F., Dong, F., Liu, D., He, H.P., 2015. A comparative study of tubular halloysite and platy kaolinite as carriers for the loading and release of the herbicide amitrole. Appl. Clay Sci. 114, 190–196. Tao, T., Maciel, G.E., 1998. 13C NMR study of co-contamination of clays with carbon tetrachloride and benzene. Environ. Sci. Technol. 32, 350–357. Tao, T., Yang, J.J., Maciel, G.E., 1999. Photoinduced decomposition of trichloroethylene on soil components. Environ. Sci. Technol. 33, 74–80. Tejada, M., Morillo, E., Go´mez, I., Madrid, F., Undabeyti, T., 2017. Effect of controlled release formulations of diuron and alachlorherbicides on the biochemical activity of agricultural soils. J. Hazard. Mater. 322, 334–347.

252

Surface and Interface Chemistry of Clay Minerals

Thiebault, T., Gu
egan, R., Boussafir, M., 2015. Adsorption mechanisms of emerging micropollutants with a clay mineral: case of tramadol and doxepine pharmaceutical products. J. Colloid Interface Sci. 453, 1–8. Thiebault, T., Boussafir, M., Le Forestier, L., Le Milbeau, C., Monnina, L., Gu
egan, R., 2016. Competitive adsorption of a pool of pharmaceuticals onto a raw clay mineral. RSC Adv. 6, 65257. Thiele-Bruhn, S., 2003. Pharmaceutical antibiotic compounds in soils—a review. J. Plant Nutr. Soil Sci. 166, 145–167. Tomic, Z.P., Asˇanin, D., Ðurovic, R., Ðordevic, A., Makreski, P., 2012. Near-infrared spectroscopy study for determination of adsorbed acetochlor in the organic and inorganic bentonites. Spectrochim. Acta A 98, 47–52. Tomic, Z.P., Kaluderovic, L., Nikolic, N., Markovic, S., Makreski, P., 2016. Thermal investigation of acetochlor adsorption on inorganic and organic-modified montmorillonite. J. Therm. Anal. Calorim. 123, 2313–2319. Torrents, A., Jayasundera, S., 1997. The sorption of nonionic pesticides onto clays and the influence of natural organic carbon. Chemosphere 35 (7), 1549–1565. Tunega, D., Gerzabek, M.H., Haberhauer, G., Lischka, H., 2007. Formation of 2,4-complexes on montmorillonites—an ab initio molecular dynamics study. Eur. J. Soil Sci. 58, 680–691. Undabeytia, T., Gala´n-Jim
enez, M.C., Go´mez-Pantoja, E., Va´zquez, J., Casal, B., Bergaya, F., Morillo, E., 2013. Fe-pillared clay mineral-based formulations of imazaquin for reduced leaching in soil. Appl. Clay Sci. 80–81, 382–389. Vaccari, A., 1998. Preparation and catalytic properties of cationic and anionic clays. Catal. Today 41 (1–3), 53–71. Valisko, M., Boda, D., 2007. Selective adsorption of ions with different diameter and valence at highly charged interfaces. J. Phys. Chem. C 111 (43), 15575–15585. Vimonses, V., Lei, S.M., Jin, B., Chowd, C.W.K., Saint, C., 2009. Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials. Chem. Eng. J 148, 354–364. Wang, C.-J., Li, Z.H., Jiang, W.-T., 2011. Adsorption of ciprofloxacin on 2:1 dioctahedral clay minerals. Appl. Clay Sci. 53, 723–728. Wang, Q., Zhang, J.P., Wang, A.Q., 2013. Alkali activation of halloysite for adsorption and release of ofloxacin. Appl. Surf. Sci. 287, 54–61. Wanyika, H., 2014. Controlled release of agrochemicals intercalated into montmorillonite interlayer space. Sci. World J. 2014, 656287. Weber, J.B., 1966. Molecular structure and pH effects on the adsorption of 13 s-triazine compounds on montmorillonite clay. Am. Mineral. 51, 1657–1670. Weber, J.B., Wilkerson, G.G., Reinhardt, C.F., 2004. Calculating pesticide sorption coefficients (Kd) using selected soil properties. Chemosphere 55, 157–166. Wei, J., Furrer, G., Kaufmann, S., Schulin, R., 2001. Influence of clay minerals on the hydrolysis of carbamate pesticides. Environ. Sci. Technol. 35, 2226–2232. Werner, D., Garratt, J.A., Pigott, G., 2013. Sorption of 2,4-D and other phenoxy herbicides to soil, organic matter, and minerals. J. Soil. Sediment. 13, 129–139. Wu, Q.F., Li, Z.H., Hong, H.L., Yin, K., Tie, L.Y., 2010. Adsorption and intercalation of ciprofloxacin on montmorillonite. Appl. Clay Sci. 50, 204–211. Wu, Q.F., Li, Z.H., Hong, H.L., 2013a. Adsorption of the quinolone antibiotic nalidixic acid onto montmorillonite and kaolinite. Appl. Clay Sci. 74, 66–73. Wu, Q.F., Li, Z.H., Hong, H.L., Li, R.B., Jiang, W.-T., 2013b. Desorption of ciprofloxacin from clay mineral surfaces. Water Res. 47, 259–268.

Organic pollutant adsorption on clay minerals

253

Xiong, J., Maciel, G.E., 1999. Deuterium NMR studies of local motions of benzene adsorbed on Ca-montmorillonite. J. Phys. Chem. B 103, 5543–5549. Xu, J.C., Stucki, J.W., Wu, J., Kostka, J.E., Sims, G.K., 2001. Fate of atrazine and alachlor in redox-treated ferruginous smectite. Environ. Toxicol. Chem. 20 (12), 2717–2724. Yang, R., Li, D.W., Li, A.M., Yang, H., 2018. Adsorption properties and mechanisms of palygorskite for removal of various ionic dyes from water. Appl. Clay Sci. 151, 20–28. Yariv, S., Lurie, D., 1971. Metachromasy in clay minerals. Part I. Sorption of methylene-blue by montmorillonite. Isr. J. Chem. 9, 537–552. Ye, P., Lemley, A.T., 2008. Adsorption effect on the degradation of carbaryl, mecoprop, and paraquat by anodic fenton treatment in an SWy-2 montmorillonite clay slurry. J. Agric. Food Chem. 56, 10200–10207. Yuan, G.D., Theng, B.K.G., Churchman, G.J., Gates, W.P., 2013. Clays and clay minerals for pollution control. In: Lagaly, F.B.a.G. (Ed.), Handbook of Clay Science Part B: Techniques and Applications. Elsevier, Amsterdam. Yusoff, S.N.M., Kamari, A., Aljafree, N.F.A., 2016. A review of materials used as carrier agents in pesticide formulations. Int. J. Environ. Sci. Technol. 13 (12), 2977–2994. Zarpon, L., Abate, G., dos Santos, L.B.O., Mas, J.C., 2006. Montmorillonite as an adsorbent for extraction and concentration of atrazine, propazine, deethylatrazine, deisopropylatrazine and hydroxyatrazine. Anal. Chim. Acta 579, 81–87. Zhang, W.H., Ding, Y.J., Boyd, S.A., Teppen, B.J., Li, H., 2010. Sorption and desorption of carbamazepine from water by smectite clays. Chemosphere 81, 954–960. Zhu, R.L., Chen, Q.Z., Liu, H.Y., Ge, F., Zhu, L.F., Zhua, J.X., He, H.P., 2014. Montmorillonite as a multifunctional adsorbent can simultaneously remove crystal violet, cetyltrimethylammonium, and 2-naphthol from water. Appl. Clay Sci. 88–89, 33–38.